diff --git a/docs/index.md b/docs/index.md
index 3dd79d1e..3498746d 100644
--- a/docs/index.md
+++ b/docs/index.md
@@ -17,6 +17,7 @@ Use the guides below to set up the app, connect your library tools, and understa
 - [OIDC](oidc.md)
 - [API Access](api-access.md)
 - [URL Search Parameters](url-search-parameters.md)
+- [Library Check](library-check.md)
 - [Custom Scripts](custom-scripts.md)
 
 ## Help
diff --git a/docs/library-check.md b/docs/library-check.md
new file mode 100644
index 00000000..33a8f573
--- /dev/null
+++ b/docs/library-check.md
@@ -0,0 +1,43 @@
+# Library Check
+
+Shelfmark can mark search results you already own, so you do not download a second copy
+of a book that is already on your shelf. The check is read only and off by default.
+
+## Calibre
+
+Point Shelfmark at the `metadata.db` of a Calibre library (Calibre, Calibre-Web,
+Calibre-Web-Automated, anything that keeps the standard Calibre format) and it reads the
+database directly. No HTTP call, no API token, and nothing is ever written back.
+
+1. Mount the library folder into the container read only, for example
+   `/path/to/calibre-library:/calibre-library:ro`. Mount the folder rather than the file
+   so the `-wal` and `-shm` sidecars are visible, otherwise a library that is being
+   written to can read as out of date.
+2. In **Settings, General**, turn on **Mark books already in your Calibre library**.
+3. Leave **Calibre metadata.db path** at `/calibre-library/metadata.db` unless you mounted
+   it somewhere else.
+4. Press **Test Calibre library**. It reports how many books it indexed.
+
+A result that matches the library then carries an **In library** badge in the card, list
+and compact views, and in the details dialog.
+
+## How a match is decided
+
+In order of confidence:
+
+1. An external id the metadata provider and the library agree on.
+2. An ISBN, compared in both ISBN-10 and ISBN-13 form.
+3. Fuzzy title tokens plus the author surname, the same rule the rest of the app uses for
+   book matching.
+
+## Behaviour worth knowing
+
+- **It fails open.** If the database cannot be read, Shelfmark logs a warning, reuses the
+  last successful read if it has one, and otherwise treats the book as not owned. A broken
+  path degrades the badge, it never blocks a search.
+- **Results are cached** for ten minutes, and refreshed early when the database file
+  changes, so a large library costs one read rather than one per search.
+- **Ebooks only.** A Calibre library holds ebooks, so the badge answers for ebooks. The
+  provider interface in `shelfmark/core/library_providers/` takes more libraries: add a
+  module with the `LibraryProvider` shape and list it in `all_providers()`. Nothing above
+  that function knows which libraries exist.
diff --git a/shelfmark/config/settings.py b/shelfmark/config/settings.py
index 476e9d54..4b70b14d 100644
--- a/shelfmark/config/settings.py
+++ b/shelfmark/config/settings.py
@@ -389,6 +389,13 @@ def _clear_metadata_cache(current_values: dict) -> dict:
         }
 
 
+def _test_calibre_library(current_values: dict[str, Any] | None = None) -> dict[str, Any]:
+    """Action-button callback: read the Calibre database and count the books."""
+    from shelfmark.core import library_index
+
+    return library_index.test_connection("calibre", current_values)
+
+
 @register_settings("general", "General", icon="settings", order=0)
 def general_settings() -> list[SettingsField]:
     """Core application settings."""
@@ -412,6 +419,31 @@ def general_settings() -> list[SettingsField]:
             description="Adds a separate navigation button for your audiobook library (Audiobookshelf, Plex, etc). When both URLs are set, icons are shown instead of text.",
             placeholder="http://audiobookshelf:8080",
         ),
+        CheckboxField(
+            key="LIBRARY_CHECK_CALIBRE_ENABLED",
+            label="Mark books already in your Calibre library",
+            description=(
+                "Read the Calibre metadata.db and mark search results you already own, so you "
+                "do not download a second copy. Read only, nothing is written to the library."
+            ),
+            default=False,
+        ),
+        TextField(
+            key="CALIBRE_LIBRARY_DB_PATH",
+            label="Calibre metadata.db path",
+            description=(
+                "Path to metadata.db as seen from inside the Shelfmark container. Mount the "
+                "Calibre library folder read-only, e.g. /path/to/calibre-library:/calibre-library:ro."
+            ),
+            default="/calibre-library/metadata.db",
+            placeholder="/calibre-library/metadata.db",
+        ),
+        ActionButton(
+            key="test_calibre_library",
+            label="Test Calibre library",
+            description="Check that Shelfmark can read the Calibre database and count the books.",
+            callback=_test_calibre_library,
+        ),
         HeadingField(
             key="search_defaults_heading",
             title="Default Search Filters",
diff --git a/shelfmark/core/library_index.py b/shelfmark/core/library_index.py
new file mode 100644
index 00000000..26d25db8
--- /dev/null
+++ b/shelfmark/core/library_index.py
@@ -0,0 +1,192 @@
+"""Library ownership check: is this book already in one of the user's libraries?
+
+Each ``LibraryProvider`` (see ``library_providers``) indexes its library into
+``LibraryEntry`` rows. This module caches those rows per provider, routes a lookup to
+the providers that hold the requested content type, and matches by identifier first
+(the book's id in the same metadata provider, ISBN in either 10 or 13 form), otherwise
+fuzzy title-token overlap plus the author surname (``text_match``) - the same rule the
+release matcher in ``auto_download`` uses.
+
+Fail-open by design: a disabled or unreachable library never stalls the pipeline.
+``is_in_library`` answers False (or from the stale cache) and logs a warning.
+"""
+
+from __future__ import annotations
+
+import threading
+import time
+from dataclasses import dataclass
+from typing import TYPE_CHECKING, Any
+
+from shelfmark.core.library_providers import all_providers
+from shelfmark.core.logger import setup_logger
+from shelfmark.core.text_match import (
+    COLLECTION_MARKERS,
+    author_surname,
+    extra_work_tokens,
+    isbn_variants,
+    title_tokens_match,
+)
+
+if TYPE_CHECKING:
+    from collections.abc import Mapping
+
+    from shelfmark.core.library_providers import LibraryEntry, LibraryProvider
+    from shelfmark.metadata_providers import BookMetadata
+
+logger = setup_logger(__name__)
+
+_CACHE_TTL_SECONDS = 600  # Re-index a library at most every 10 minutes unless it changed.
+
+
+@dataclass
+class _CacheSlot:
+    entries: list[LibraryEntry] | None = None
+    fetched_at: float = 0.0
+    fingerprint: object | None = None
+
+
+_lock = threading.Lock()
+_cache: dict[str, _CacheSlot] = {}
+
+
+def _slot(provider_name: str) -> _CacheSlot:
+    with _lock:
+        return _cache.setdefault(provider_name, _CacheSlot())
+
+
+def _store(provider_name: str, entries: list[LibraryEntry], fingerprint: object | None) -> None:
+    slot = _slot(provider_name)
+    with _lock:
+        slot.entries = entries
+        slot.fetched_at = time.monotonic()
+        slot.fingerprint = fingerprint
+
+
+def _entries_for(provider: LibraryProvider) -> list[LibraryEntry]:
+    """Cached entries for one provider, re-indexed past the TTL or when the library changed."""
+    slot = _slot(provider.name)
+    try:
+        fingerprint = provider.fingerprint()
+        with _lock:
+            cached = slot.entries
+            fresh = time.monotonic() - slot.fetched_at < _CACHE_TTL_SECONDS
+            unchanged = fingerprint == slot.fingerprint
+        if cached is not None and fresh and unchanged:
+            return cached
+        entries = provider.fetch_entries()
+    except Exception as exc:  # noqa: BLE001 - any failure must fail open
+        logger.warning("library check: %s unavailable (%s); failing open", provider.describe(), exc)
+        with _lock:
+            return slot.entries or []  # Use the stale cache if we have one.
+
+    _store(provider.name, entries, fingerprint)
+    logger.info("library check: indexed %d %s item(s)", len(entries), provider.display_name)
+    return entries
+
+
+def _enabled_providers(content_type: str | None) -> list[LibraryProvider]:
+    return [
+        provider
+        for provider in all_providers()
+        if provider.is_enabled()
+        and (content_type is None or content_type in provider.content_types)
+    ]
+
+
+def any_provider_enabled() -> bool:
+    return any(provider.is_enabled() for provider in all_providers())
+
+
+def match_entries(book: BookMetadata, entries: list[LibraryEntry]) -> str | None:
+    """How ``book`` is held: ``"owned"``, ``"collection"``, or None when it is not.
+
+    ``"collection"`` means a shelf title that bundles several works matched, e.g. an
+    omnibus. The reader has the book, but saying so plainly would misdescribe what is
+    on the shelf.
+    """
+    if not entries:
+        return None
+
+    external_id = (book.provider, str(book.provider_id)) if book.provider_id else None
+    book_isbns = isbn_variants(book.isbn_13) | isbn_variants(book.isbn_10)
+    for entry in entries:
+        if external_id in entry.external_ids or entry.isbns & book_isbns:
+            return "owned"
+
+    title = book.search_title or book.title
+    surname = author_surname(book.search_author or (book.authors[0] if book.authors else ""))
+    collection: str | None = None
+    for entry in entries:
+        if not title_tokens_match(title, set(entry.tokens)):
+            continue
+        if surname is not None and surname not in entry.tokens:
+            continue
+        # A shorter search title is a subset of every longer shelf title sharing its
+        # words, so "Dune" matched "Dune Messiah". Words the shelf adds that name
+        # another work disqualify it; packaging words do not.
+        if extra_work_tokens(set(entry.title_tokens), title, set(entry.context_tokens)):
+            continue
+        if entry.title_tokens & COLLECTION_MARKERS:
+            collection = "collection"
+            continue
+        return "owned"
+
+    return collection
+
+
+def book_matches_entries(book: BookMetadata, entries: list[LibraryEntry]) -> bool:
+    """True if ``book`` is on the shelf at all, however it is packaged."""
+    return match_entries(book, entries) is not None
+
+
+def is_in_library(book: BookMetadata, content_type: str | None = None) -> bool:
+    """True if an enabled library holding ``content_type`` already has ``book`` (fail-open).
+
+    ``content_type`` None consults every enabled provider.
+    """
+    return any(
+        book_matches_entries(book, _entries_for(provider))
+        for provider in _enabled_providers(content_type)
+    )
+
+
+def _holding(book: BookMetadata, content_type: str) -> str | None:
+    """Strongest holding across the enabled libraries for one content type."""
+    kinds = {
+        match_entries(book, _entries_for(provider)) for provider in _enabled_providers(content_type)
+    }
+    if "owned" in kinds:
+        return "owned"
+    return "collection" if "collection" in kinds else None
+
+
+def ownership(book: BookMetadata) -> dict[str, str | None] | None:
+    """Per-format holding for the UI: ``{"ebook": "owned" | "collection" | None}``.
+
+    Only content types with an enabled library are reported; None when no library
+    check is enabled at all. Fail-open like ``is_in_library``.
+    """
+    result = {
+        content_type: _holding(book, content_type)
+        for content_type in ("ebook", "audiobook")
+        if _enabled_providers(content_type)
+    }
+    return result or None
+
+
+def test_connection(provider_name: str, current_values: Mapping[str, Any] | None) -> dict[str, Any]:
+    """Settings action: index one library now and report the item count.
+
+    ``current_values`` are the unsaved form values, so the button works before Save.
+    """
+    provider = next((p for p in all_providers(current_values) if p.name == provider_name), None)
+    if provider is None:
+        return {"success": False, "message": f"Unknown library provider: {provider_name}"}
+    try:
+        fingerprint = provider.fingerprint()
+        entries = provider.fetch_entries()
+    except Exception as exc:  # noqa: BLE001 - surface any error to the user
+        return {"success": False, "message": f"{provider.describe()}: {exc}"}
+    _store(provider.name, entries, fingerprint)
+    return {"success": True, "message": f"{provider.describe()}: indexed {len(entries)} item(s)."}
diff --git a/shelfmark/core/library_providers/__init__.py b/shelfmark/core/library_providers/__init__.py
new file mode 100644
index 00000000..16a12789
--- /dev/null
+++ b/shelfmark/core/library_providers/__init__.py
@@ -0,0 +1,73 @@
+"""Library ownership providers.
+
+Each provider indexes one library the user already owns into matchable
+``LibraryEntry`` rows. ``shelfmark.core.library_index`` caches those rows per provider
+and answers whether a requested book is already on the shelf.
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import TYPE_CHECKING, Any, Protocol
+
+if TYPE_CHECKING:
+    from collections.abc import Mapping
+
+
+@dataclass(frozen=True)
+class LibraryEntry:
+    """Normalized, matchable representation of one library item."""
+
+    tokens: frozenset[str]
+    isbns: frozenset[str]
+    asins: frozenset[str]
+    # (metadata provider name, id in that provider) pairs, e.g. ("hardcover", "446681").
+    external_ids: frozenset[tuple[str, str]] = frozenset()
+    # Title tokens alone, without the author and series that `tokens` also carries.
+    # The matcher needs them to tell "the shelf title says more than the search did"
+    # from "the shelf entry merely has an author", which decides whether a shorter
+    # search title is the same book or a different one in the same series.
+    title_tokens: frozenset[str] = frozenset()
+    # Series and author words. Shelf titles often bake these in ("Alex Cross 25:
+    # Cross Kill"), so the matcher must not read them as naming a different work.
+    context_tokens: frozenset[str] = frozenset()
+
+
+class LibraryProvider(Protocol):
+    """One owned-book library the ownership check can consult."""
+
+    name: str
+    display_name: str
+    content_types: frozenset[str]
+
+    def is_enabled(self) -> bool: ...
+
+    def fetch_entries(self) -> list[LibraryEntry]: ...
+
+    def describe(self) -> str: ...
+
+    def fingerprint(self) -> object | None:
+        """Cheap change token (e.g. a file mtime); None when the library has none."""
+        ...
+
+
+def setting(
+    config: Any, key: str, default: object = "", overrides: Mapping[str, Any] | None = None
+) -> object:
+    """Read ``key`` from ``config``, preferring a non-empty unsaved form value in ``overrides``."""
+    if overrides:
+        value = overrides.get(key)
+        if value not in (None, ""):
+            return value
+    return config.get(key, default)
+
+
+def all_providers(overrides: Mapping[str, Any] | None = None) -> list[LibraryProvider]:
+    """Concrete providers, in a fixed order. ``overrides`` are unsaved form values.
+
+    Adding a library means writing a module with the :class:`LibraryProvider` shape and
+    listing it here. Nothing above this function knows which libraries exist.
+    """
+    from shelfmark.core.library_providers.calibre import CalibreLibrary
+
+    return [CalibreLibrary(overrides)]
diff --git a/shelfmark/core/library_providers/calibre.py b/shelfmark/core/library_providers/calibre.py
new file mode 100644
index 00000000..1c0611af
--- /dev/null
+++ b/shelfmark/core/library_providers/calibre.py
@@ -0,0 +1,161 @@
+"""Calibre library provider.
+
+Reads a Calibre ``metadata.db`` (plain Calibre, Calibre-Web or Calibre-Web Automated)
+read-only and indexes each book's title, authors, series and identifiers.
+
+The database is opened with ``mode=ro`` first: on a WAL-mode library that sees the
+un-checkpointed writes as long as the ``-wal``/``-shm`` files beside it are visible.
+When that open fails (typically a read-only mount where SQLite cannot create those
+files, or a locked database) it is retried once with ``immutable=1``, a snapshot that
+can lag until Calibre next checkpoints. The file mtime is the change fingerprint, so a
+write or checkpoint refreshes the index ahead of the cache TTL.
+"""
+
+from __future__ import annotations
+
+import re
+import sqlite3
+from contextlib import closing
+from pathlib import Path
+from typing import TYPE_CHECKING, Any
+
+from shelfmark.core.config import config as app_config
+from shelfmark.core.library_providers import LibraryEntry, setting
+from shelfmark.core.logger import setup_logger
+from shelfmark.core.text_match import isbn_variants, tokens
+
+if TYPE_CHECKING:
+    from collections.abc import Mapping
+
+logger = setup_logger(__name__)
+
+# "(Alex Cross Series #11)", "[Illustrated Edition]" and friends.
+_PARENTHETICAL = re.compile(r"[(\[][^)\]]*[)\]]")
+
+_DEFAULT_DB_PATH = "/calibre-library/metadata.db"
+_BUSY_TIMEOUT_SECONDS = 5
+
+_ISBN_TYPES = frozenset({"isbn", "isbn13", "isbn-13", "isbn10", "isbn-10"})
+_ASIN_TYPES = frozenset({"amazon", "mobi-asin", "asin"})
+# Calibre identifier type -> Shelfmark metadata provider name.
+_EXTERNAL_ID_TYPES = {
+    "hardcover-id": "hardcover",
+    "google": "googlebooks",
+    "openlibrary": "openlibrary",
+    "olid": "openlibrary",
+}
+
+_BOOKS_SQL = "SELECT id, title FROM books"
+_AUTHORS_SQL = "SELECT l.book, a.name FROM books_authors_link l JOIN authors a ON a.id = l.author"
+_SERIES_SQL = "SELECT l.book, s.name FROM books_series_link l JOIN series s ON s.id = l.series"
+_IDENTIFIERS_SQL = "SELECT book, type, val FROM identifiers"
+
+
+def _db_path(overrides: Mapping[str, Any] | None = None) -> Path:
+    return Path(
+        str(
+            setting(app_config, "CALIBRE_LIBRARY_DB_PATH", _DEFAULT_DB_PATH, overrides)
+            or _DEFAULT_DB_PATH
+        )
+    )
+
+
+def _probe(conn: sqlite3.Connection) -> sqlite3.Connection:
+    """Force the first read so open failures surface here rather than mid-query."""
+    try:
+        conn.execute("PRAGMA schema_version").fetchone()
+    except sqlite3.Error:
+        conn.close()
+        raise
+    return conn
+
+
+def _connect(path: Path) -> sqlite3.Connection:
+    uri = f"{path.absolute().as_uri()}?mode=ro"
+    try:
+        conn = _probe(sqlite3.connect(uri, uri=True, timeout=_BUSY_TIMEOUT_SECONDS))
+    except sqlite3.OperationalError as exc:
+        logger.info(
+            "library check: %s cannot be read in place (%s); using an immutable snapshot, "
+            "which may lag until Calibre checkpoints the library",
+            path,
+            exc,
+        )
+        conn = _probe(sqlite3.connect(f"{uri}&immutable=1", uri=True))
+    return conn
+
+
+def _read_entries(conn: sqlite3.Connection) -> list[LibraryEntry]:
+    titles: dict[int, str] = dict(conn.execute(_BOOKS_SQL).fetchall())
+    series: dict[int, str] = dict(conn.execute(_SERIES_SQL).fetchall())
+
+    authors: dict[int, list[str]] = {}
+    for book_id, name in conn.execute(_AUTHORS_SQL):
+        authors.setdefault(book_id, []).append(name)
+
+    isbns: dict[int, set[str]] = {}
+    asins: dict[int, set[str]] = {}
+    external_ids: dict[int, set[tuple[str, str]]] = {}
+    for book_id, id_type, raw in conn.execute(_IDENTIFIERS_SQL):
+        value = str(raw or "").strip()
+        kind = str(id_type or "").strip().lower()
+        if not value:
+            continue
+        if kind in _ISBN_TYPES:
+            isbns.setdefault(book_id, set()).update(isbn_variants(value))
+        elif kind in _ASIN_TYPES:
+            asins.setdefault(book_id, set()).add(value.upper())
+        elif provider := _EXTERNAL_ID_TYPES.get(kind):
+            external_ids.setdefault(book_id, set()).add((provider, value))
+
+    entries: list[LibraryEntry] = []
+    for book_id, title in titles.items():
+        # A trailing parenthetical is series or edition metadata by convention rather
+        # than part of the work name, and Calibre users often put it there instead of
+        # in the series field. It stays in the recall set below and is dropped from the
+        # title set, which is what decides whether the shelf holds a different book.
+        title_tok = set(tokens(_PARENTHETICAL.sub(" ", title)))
+        context_tok = set(tokens(series.get(book_id)))
+        for name in authors.get(book_id, ()):
+            context_tok |= set(tokens(name))
+        tok = title_tok | context_tok
+        entries.append(
+            LibraryEntry(
+                frozenset(tok),
+                frozenset(isbns.get(book_id, ())),
+                frozenset(asins.get(book_id, ())),
+                frozenset(external_ids.get(book_id, ())),
+                frozenset(title_tok),
+                frozenset(context_tok),
+            )
+        )
+    return entries
+
+
+class CalibreLibrary:
+    """Ebook ownership via a read-only Calibre ``metadata.db``."""
+
+    name = "calibre"
+    display_name = "Calibre"
+    content_types = frozenset({"ebook"})
+
+    def __init__(self, overrides: Mapping[str, Any] | None = None) -> None:
+        self._overrides = overrides
+
+    def is_enabled(self) -> bool:
+        return bool(setting(app_config, "LIBRARY_CHECK_CALIBRE_ENABLED", False, self._overrides))
+
+    def describe(self) -> str:
+        return f"Calibre library at {_db_path(self._overrides)}"
+
+    def fingerprint(self) -> float | None:
+        path = _db_path(self._overrides)
+        candidates = (path, path.with_name(f"{path.name}-wal"))
+        return max((p.stat().st_mtime for p in candidates if p.exists()), default=None)
+
+    def fetch_entries(self) -> list[LibraryEntry]:
+        path = _db_path(self._overrides)
+        if not path.is_file():
+            raise FileNotFoundError(f"No Calibre database at {path}")
+        with closing(_connect(path)) as conn:
+            return _read_entries(conn)
diff --git a/shelfmark/core/text_match.py b/shelfmark/core/text_match.py
new file mode 100644
index 00000000..4c66fa07
--- /dev/null
+++ b/shelfmark/core/text_match.py
@@ -0,0 +1,171 @@
+"""Shared text-normalization + fuzzy token-matching helpers for book matching.
+
+Used by the library ownership check (``library_index``) and its providers, so every
+library matches titles, authors and ISBNs the same way.
+"""
+
+from __future__ import annotations
+
+import re
+
+DEFAULT_TITLE_MATCH_THRESHOLD = 0.85
+
+# Short/common words that add noise to title token matching.
+STOPWORDS = frozenset(
+    {
+        "a",
+        "an",
+        "the",
+        "of",
+        "and",
+        "or",
+        "to",
+        "in",
+        "on",
+        "for",
+        "with",
+        "is",
+        "by",
+    }
+)
+
+
+def tokens(text: str | None) -> list[str]:
+    """Lowercase alphanumeric tokens from arbitrary text."""
+    if not text:
+        return []
+    return [tok for tok in re.split(r"[^a-z0-9]+", text.lower()) if tok]
+
+
+def significant_tokens(text: str | None) -> list[str]:
+    """Tokens with stopwords and 1-char noise removed."""
+    return [tok for tok in tokens(text) if len(tok) >= 2 and tok not in STOPWORDS]
+
+
+def author_surname(author: str | None) -> str | None:
+    """Return the most distinctive author token (the surname), or None."""
+    value = author or ""
+    if "," in value:  # "Last, First" -> keep the "Last" portion
+        value = value.split(",")[0]
+    toks = significant_tokens(value)
+    return toks[-1] if toks else None
+
+
+def title_tokens_match(
+    title: str | None,
+    haystack_tokens: set[str],
+    threshold: float = DEFAULT_TITLE_MATCH_THRESHOLD,
+) -> bool:
+    """True when enough significant title tokens appear in ``haystack_tokens``."""
+    title_toks = significant_tokens(title)
+    if not title_toks:
+        return False
+    present = sum(1 for tok in title_toks if tok in haystack_tokens)
+    return (present / len(title_toks)) >= threshold
+
+
+# Words marking a title that bundles several works. A shelf entry carrying one of
+# these holds the searched book, but as part of something larger, which is worth
+# telling the reader apart from owning it on its own.
+COLLECTION_MARKERS = frozenset(
+    {
+        "omnibus",
+        "collection",
+        "complete",
+        "boxed",
+        "boxset",
+        "box",
+        "set",
+        "bundle",
+        "anthology",
+        "compendium",
+        "trilogy",
+        "duology",
+        "books",
+        "volumes",
+        "vols",
+    }
+)
+
+# Words marking a different printing of the same work, which changes nothing about
+# whether the reader owns it.
+EDITION_MARKERS = frozenset(
+    {
+        "edition",
+        "editions",
+        "series",
+        "book",
+        "vol",
+        "volume",
+        "illustrated",
+        "annotated",
+        "unabridged",
+        "abridged",
+        "deluxe",
+        "revised",
+        "reissue",
+        "anniversary",
+    }
+)
+
+# Neither kind is evidence that the shelf holds a different book.
+PACKAGING_MARKERS = COLLECTION_MARKERS | EDITION_MARKERS
+
+
+def extra_work_tokens(
+    shelf_title_tokens: set[str],
+    search_title: str | None,
+    context_tokens: set[str] | None = None,
+) -> set[str]:
+    """Significant words the shelf title adds that suggest a different work.
+
+    Packaging words are ignored, since "Illustrated Edition" and "Books 1-6" describe
+    the same work differently wrapped, while "Messiah" or "Chapter Two" name another
+    Only alphabetic words of three or more characters count, so volume numbers and
+    structural words do not make a shelf entry look like a different book.
+
+    ``context_tokens`` are the entry's series and author words, which shelf titles
+    often repeat ("Alex Cross 25: Cross Kill") without meaning another book.
+    """
+    search_toks = set(tokens(search_title)) | (context_tokens or set())
+    return {
+        tok
+        for tok in shelf_title_tokens
+        # Only a real word counts. Numbers are left alone deliberately: a shelf title
+        # carries volume numbers far more often than it names a numbered sequel, and
+        # wrongly claiming ownership is the costlier mistake, since the sync then
+        # never fetches the book. "Persepolis 2" is the case this concedes.
+        if tok.isalpha()
+        and len(tok) >= 3
+        and tok not in STOPWORDS
+        and tok not in PACKAGING_MARKERS
+        and tok not in search_toks
+    }
+
+
+def normalize_isbn(value: object) -> str:
+    """Normalize an ISBN to comparable form (digits + trailing X, uppercased)."""
+    if not value:
+        return ""
+    return re.sub(r"[^0-9xX]", "", str(value)).upper()
+
+
+def _isbn10_to_isbn13(isbn10: str) -> str | None:
+    """ISBN-13 form of a normalized ISBN-10: 978 prefix plus a recomputed check digit."""
+    core = isbn10[:9]
+    if len(isbn10) != 10 or not core.isdigit():
+        return None
+    digits = f"978{core}"
+    total = sum(int(d) * (1 if i % 2 == 0 else 3) for i, d in enumerate(digits))
+    return f"{digits}{(10 - total % 10) % 10}"
+
+
+def isbn_variants(value: object) -> frozenset[str]:
+    """Comparable forms of an ISBN: normalized, plus the ISBN-13 form of an ISBN-10."""
+    isbn = normalize_isbn(value)
+    if not isbn:
+        return frozenset()
+    variants = {isbn}
+    if len(isbn) == 10 and (isbn13 := _isbn10_to_isbn13(isbn)):
+        variants.add(isbn13)
+    return frozenset(variants)
diff --git a/shelfmark/main.py b/shelfmark/main.py
index e1936cfa..e52cc1f8 100644
--- a/shelfmark/main.py
+++ b/shelfmark/main.py
@@ -2700,6 +2700,8 @@ def api_metadata_search() -> Response | tuple[Response, int]:
             if book_dict.get("cover_url"):
                 cache_id = f"{book_dict['provider']}_{book_dict['provider_id']}"
                 book_dict["cover_url"] = transform_cover_url(book_dict["cover_url"], cache_id)
+        for book, book_dict in zip(search_result.books, books_data, strict=True):
+            _attach_library_ownership(book, book_dict)
 
         response_data = {
             "books": books_data,
@@ -2760,6 +2762,15 @@ def api_metadata_field_options() -> Response:
         return jsonify({"options": []})
 
 
+def _attach_library_ownership(book: Any, book_dict: dict[str, Any]) -> None:
+    """Add per-format ownership under ``library`` when a library check is enabled."""
+    from shelfmark.core import library_index
+
+    owned = library_index.ownership(book)
+    if owned is not None:
+        book_dict["library"] = owned
+
+
 def _resolve_metadata_provider(provider_name: str) -> MetadataProvider:
     """Validate, instantiate and return a ready metadata provider.
 
@@ -2808,6 +2819,7 @@ def api_metadata_book(provider: str, book_id: str) -> Response | tuple[Response,
             return jsonify({"error": "Book not found"}), 404
 
         book_dict = asdict(book)
+        _attach_library_ownership(book, book_dict)
 
         # Transform cover_url to local proxy URL when caching is enabled
         from shelfmark.core.utils import transform_cover_url
@@ -3114,6 +3126,7 @@ def api_releases() -> Response | tuple[Response, int]:
 
         # Convert book to dict and transform cover_url
         book_dict = asdict(book)
+        _attach_library_ownership(book, book_dict)
         from shelfmark.core.utils import transform_cover_url
 
         if book_dict.get("cover_url"):
diff --git a/src/frontend/src/components/DetailsModal.tsx b/src/frontend/src/components/DetailsModal.tsx
index ad5b4af2..d5d598a2 100644
--- a/src/frontend/src/components/DetailsModal.tsx
+++ b/src/frontend/src/components/DetailsModal.tsx
@@ -9,6 +9,7 @@ import { isMetadataBook } from '../types';
 import { bookSupportsTargets } from '../utils/bookTargetLoader';
 import { isUserCancelledError } from '../utils/errors';
 import { BookTargetDropdown } from './BookTargetDropdown';
+import { LibraryBadge, isInLibrary } from './shared';
 
 interface DetailsModalProps {
   book: Book | null;
@@ -292,6 +293,13 @@ export const DetailsModal = ({
                     ))}
                 </div>
 
+                {isMetadata && isInLibrary(book.library) && (
+                  <div className={`${infoCardClass} space-y-1`}>
+                    <p className={infoLabelClass}>Library</p>
+                    <LibraryBadge library={book.library} />
+                  </div>
+                )}
+
                 {/* ISBN - Universal mode only */}
                 {isMetadata && (book.isbn_13 || book.isbn_10) && (
                   <div className={`${infoCardClass} space-y-1`}>
diff --git a/src/frontend/src/components/resultsViews/CardView.tsx b/src/frontend/src/components/resultsViews/CardView.tsx
index e20f5917..9ecbd7b5 100644
--- a/src/frontend/src/components/resultsViews/CardView.tsx
+++ b/src/frontend/src/components/resultsViews/CardView.tsx
@@ -6,7 +6,7 @@ import { getDownloadsCount } from '../../types';
 import { bookSupportsTargets } from '../../utils/bookTargetLoader';
 import { BookActionButton } from '../BookActionButton';
 import { BookTargetDropdown } from '../BookTargetDropdown';
-import { DisplayFieldBadges } from '../shared';
+import { DisplayFieldBadges, LibraryBadge } from '../shared';
 
 const SkeletonLoader = () => (
   <div className="h-full w-full animate-pulse bg-linear-to-r from-gray-300 via-gray-200 to-gray-300 dark:from-gray-700 dark:via-gray-600 dark:to-gray-700" />
@@ -99,6 +99,7 @@ export const CardView = ({
               #{book.series_position}
             </div>
           )}
+          <LibraryBadge library={book.library} overlay className="absolute top-2 right-2 z-10" />
           {book.preview && !imageError ? (
             <>
               {!imageLoaded && (
diff --git a/src/frontend/src/components/resultsViews/CompactView.tsx b/src/frontend/src/components/resultsViews/CompactView.tsx
index 6d074c6d..217d9ecb 100644
--- a/src/frontend/src/components/resultsViews/CompactView.tsx
+++ b/src/frontend/src/components/resultsViews/CompactView.tsx
@@ -6,7 +6,7 @@ import { getDownloadsCount } from '../../types';
 import { bookSupportsTargets } from '../../utils/bookTargetLoader';
 import { BookActionButton } from '../BookActionButton';
 import { BookTargetDropdown } from '../BookTargetDropdown';
-import { DisplayFieldBadges, DisplayFieldIcon } from '../shared';
+import { DisplayFieldBadges, DisplayFieldIcon, LibraryBadge } from '../shared';
 
 const SkeletonLoader = () => (
   <div className="h-full w-full animate-pulse bg-linear-to-r from-gray-300 via-gray-200 to-gray-300 dark:from-gray-700 dark:via-gray-600 dark:to-gray-700" />
@@ -99,6 +99,7 @@ export const CompactView = ({
               #{book.series_position}
             </div>
           )}
+          <LibraryBadge library={book.library} overlay className="absolute top-2 right-2 z-10" />
           {book.preview && !imageError ? (
             <>
               {!imageLoaded && (
diff --git a/src/frontend/src/components/resultsViews/ListView.tsx b/src/frontend/src/components/resultsViews/ListView.tsx
index e457dcf7..f2e68ebe 100644
--- a/src/frontend/src/components/resultsViews/ListView.tsx
+++ b/src/frontend/src/components/resultsViews/ListView.tsx
@@ -7,7 +7,7 @@ import { bookSupportsTargets } from '../../utils/bookTargetLoader';
 import { getFormatColor, getLanguageColor } from '../../utils/colorMaps';
 import { BookActionButton } from '../BookActionButton';
 import { BookTargetDropdown } from '../BookTargetDropdown';
-import { DisplayFieldIcon, DisplayFieldBadge } from '../shared';
+import { DisplayFieldIcon, DisplayFieldBadge, LibraryBadge } from '../shared';
 
 interface ListViewProps {
   books: Book[];
@@ -203,6 +203,7 @@ export const ListView = ({
                     {book.author || 'Unknown author'}
                     {book.year && <span className="sm:hidden"> • {book.year}</span>}
                   </p>
+                  <LibraryBadge library={book.library} className="mt-0.5" />
                 </div>
 
                 {/* Mobile universal mode info */}
diff --git a/src/frontend/src/components/shared/LibraryBadge.tsx b/src/frontend/src/components/shared/LibraryBadge.tsx
new file mode 100644
index 00000000..104b8bda
--- /dev/null
+++ b/src/frontend/src/components/shared/LibraryBadge.tsx
@@ -0,0 +1,50 @@
+import type { LibraryOwnership } from '../../types';
+
+/** True when the library check reports this book held in any format. */
+export function isInLibrary(library?: LibraryOwnership | null): boolean {
+  if (!library) return false;
+  return Object.values(library).some((holding) => holding === 'owned' || holding === 'collection');
+}
+
+/** True when every format that holds it holds it inside a larger volume. */
+export function isCollectionOnly(library?: LibraryOwnership | null): boolean {
+  if (!isInLibrary(library)) return false;
+  return !Object.values(library ?? {}).includes('owned');
+}
+
+interface LibraryBadgeProps {
+  library?: LibraryOwnership | null;
+  /** Solid pill for use over cover art; otherwise a tinted inline pill. */
+  overlay?: boolean;
+  className?: string;
+}
+
+/** "Already in your library" badge. Renders nothing when the book is not held. */
+export function LibraryBadge({ library, overlay = false, className = '' }: LibraryBadgeProps) {
+  if (!isInLibrary(library)) return null;
+
+  const collectionOnly = isCollectionOnly(library);
+  const label = collectionOnly ? 'In your library, inside a collection' : 'Already in your library';
+  const text = collectionOnly ? 'In a collection' : 'In library';
+
+  const pill = overlay
+    ? 'rounded-md border border-sky-700 bg-sky-600 px-1.5 py-0.5 text-[10px] font-bold text-white'
+    : 'rounded-md bg-sky-600/15 px-1.5 py-0.5 text-[10px] font-semibold text-sky-700 dark:text-sky-300';
+  const style = overlay
+    ? { boxShadow: '0 2px 8px rgba(0, 0, 0, 0.4), 0 1px 3px rgba(0, 0, 0, 0.3)' }
+    : undefined;
+
+  return (
+    <span
+      className={`flex w-fit items-center gap-0.5 ${pill} ${className}`}
+      style={style}
+      title={label}
+      aria-label={label}
+    >
+      <svg className="h-3 w-3" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+        <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M5 13l4 4L19 7" />
+      </svg>
+      {text}
+    </span>
+  );
+}
diff --git a/src/frontend/src/components/shared/index.ts b/src/frontend/src/components/shared/index.ts
index b282eb67..c6983181 100644
--- a/src/frontend/src/components/shared/index.ts
+++ b/src/frontend/src/components/shared/index.ts
@@ -1,3 +1,4 @@
 export { DisplayFieldIcon, DisplayFieldBadge, DisplayFieldBadges } from './DisplayFieldIcon';
 export { CircularProgress } from './CircularProgress';
 export { ToggleSwitch } from './ToggleSwitch';
+export { LibraryBadge, isInLibrary, isCollectionOnly } from './LibraryBadge';
diff --git a/src/frontend/src/tests/bookTransformers.test.ts b/src/frontend/src/tests/bookTransformers.test.ts
index 1fe23079..cf1dc002 100644
--- a/src/frontend/src/tests/bookTransformers.test.ts
+++ b/src/frontend/src/tests/bookTransformers.test.ts
@@ -2,6 +2,7 @@ import { describe, it, expect } from 'vitest';
 
 import { isMetadataBook, type Release } from '../types/index';
 import {
+  transformMetadataToBook,
   transformReleaseToDirectBook,
   transformSourceRecordToBook,
 } from '../utils/bookTransformers';
@@ -84,3 +85,22 @@ describe('bookTransformers.transformSourceRecordToBook', () => {
     expect(isMetadataBook(book)).toBe(false);
   });
 });
+
+describe('bookTransformers.transformMetadataToBook', () => {
+  const metadata = {
+    provider: 'hardcover',
+    provider_id: '446681',
+    title: 'Dungeon Crawler Carl',
+    authors: ['Matt Dinniman'],
+  };
+
+  it('carries the library ownership flags through to the Book', () => {
+    const book = transformMetadataToBook({ ...metadata, library: { ebook: 'owned' as const } });
+
+    expect(book.library).toEqual({ ebook: 'owned' });
+  });
+
+  it('leaves library undefined when the API sends none', () => {
+    expect(transformMetadataToBook(metadata).library).toBeUndefined();
+  });
+});
diff --git a/src/frontend/src/tests/libraryBadge.test.ts b/src/frontend/src/tests/libraryBadge.test.ts
new file mode 100644
index 00000000..08975d2b
--- /dev/null
+++ b/src/frontend/src/tests/libraryBadge.test.ts
@@ -0,0 +1,39 @@
+import { describe, expect, it } from 'vitest';
+
+import { isCollectionOnly, isInLibrary } from '../components/shared/LibraryBadge';
+
+describe('isInLibrary', () => {
+  it('is false without a library result', () => {
+    expect(isInLibrary(undefined)).toBe(false);
+    expect(isInLibrary(null)).toBe(false);
+    expect(isInLibrary({})).toBe(false);
+  });
+
+  it('is false when no format holds it', () => {
+    expect(isInLibrary({ ebook: null })).toBe(false);
+    expect(isInLibrary({ ebook: null, audiobook: null })).toBe(false);
+  });
+
+  it('is true for a book held outright or inside a collection', () => {
+    expect(isInLibrary({ ebook: 'owned' })).toBe(true);
+    expect(isInLibrary({ ebook: 'collection' })).toBe(true);
+    expect(isInLibrary({ ebook: null, audiobook: 'owned' })).toBe(true);
+  });
+});
+
+describe('isCollectionOnly', () => {
+  it('is true only when every holding is inside a collection', () => {
+    expect(isCollectionOnly({ ebook: 'collection' })).toBe(true);
+    expect(isCollectionOnly({ ebook: 'collection', audiobook: null })).toBe(true);
+  });
+
+  it('is false when any format holds the book on its own', () => {
+    expect(isCollectionOnly({ ebook: 'owned' })).toBe(false);
+    expect(isCollectionOnly({ ebook: 'collection', audiobook: 'owned' })).toBe(false);
+  });
+
+  it('is false when the book is not held at all', () => {
+    expect(isCollectionOnly({ ebook: null })).toBe(false);
+    expect(isCollectionOnly(undefined)).toBe(false);
+  });
+});
diff --git a/src/frontend/src/types/index.ts b/src/frontend/src/types/index.ts
index 3dfb6f44..38f1a8eb 100644
--- a/src/frontend/src/types/index.ts
+++ b/src/frontend/src/types/index.ts
@@ -32,6 +32,7 @@ export interface Book {
   status_message?: string; // Detailed status message (e.g., "Trying Libgen (2/5)")
   added_time?: number; // Timestamp when added to queue
   content_type?: string; // "ebook", "audiobook", or related book subtype
+  library?: LibraryOwnership; // "already in your library" flags from the library check
   source?: string; // Release source handler (e.g., "direct_download", "prowlarr")
   source_display_name?: string; // Human-readable source name (e.g., "Direct Download")
   // Metadata provider fields (used in universal search mode)
@@ -277,6 +278,14 @@ export interface QueuedDownloadResult {
 
 export type RequestSubmissionResult = RequestRecord | QueuedDownloadResult;
 
+/** How the library holds a book, per format: owned outright, or inside a collection. */
+export type LibraryHolding = 'owned' | 'collection';
+
+export interface LibraryOwnership {
+  ebook?: LibraryHolding | null;
+  audiobook?: LibraryHolding | null;
+}
+
 export type BooksOutputMode = 'folder' | 'booklore' | 'email';
 
 export interface AppConfig {
diff --git a/src/frontend/src/utils/bookTransformers.ts b/src/frontend/src/utils/bookTransformers.ts
index 1279f732..546f5e9d 100644
--- a/src/frontend/src/utils/bookTransformers.ts
+++ b/src/frontend/src/utils/bookTransformers.ts
@@ -1,4 +1,4 @@
-import type { Book, Release } from '../types';
+import type { Book, LibraryOwnership, Release } from '../types';
 import { isRecord, isStringArray } from './objectHelpers';
 
 /**
@@ -6,6 +6,7 @@ import { isRecord, isStringArray } from './objectHelpers';
  * Used by both search and single-book endpoints.
  */
 export interface MetadataBookData {
+  library?: LibraryOwnership; // ownership flags from the library check
   provider: string;
   provider_display_name?: string;
   provider_id: string;
@@ -102,6 +103,7 @@ export function transformMetadataToBook(data: MetadataBookData): Book {
     search_author: data.search_author,
     authors: data.authors,
     titles_by_language: data.titles_by_language,
+    library: data.library,
     info: {
       ...(data.isbn_13 && { ISBN: data.isbn_13 }),
       ...(data.isbn_10 && !data.isbn_13 && { ISBN: data.isbn_10 }),
diff --git a/tests/core/test_library_index.py b/tests/core/test_library_index.py
new file mode 100644
index 00000000..74c86fbf
--- /dev/null
+++ b/tests/core/test_library_index.py
@@ -0,0 +1,421 @@
+"""Tests for the library ownership facade: matcher, routing, caching and fail-open."""
+
+from __future__ import annotations
+
+from typing import Any
+
+import pytest
+
+from shelfmark.core import library_index
+from shelfmark.core.library_providers import LibraryEntry
+from shelfmark.core.text_match import isbn_variants
+from shelfmark.metadata_providers import BookMetadata
+
+
+def _book(**overrides: Any) -> BookMetadata:
+    fields: dict[str, Any] = {
+        "provider": "hardcover",
+        "provider_id": "446681",
+        "title": "Dungeon Crawler Carl",
+        "authors": ["Matt Dinniman"],
+    }
+    fields.update(overrides)
+    return BookMetadata(**fields)
+
+
+def _entry(
+    *words: str,
+    isbns: frozenset[str] = frozenset(),
+    external_ids: frozenset[tuple[str, str]] = frozenset(),
+) -> LibraryEntry:
+    return LibraryEntry(frozenset(words), isbns, frozenset(), external_ids)
+
+
+_DCC_ENTRY = _entry("dungeon", "crawler", "carl", "matt", "dinniman")
+
+
+class _Provider:
+    def __init__(
+        self,
+        name: str,
+        content_types: set[str],
+        entries: list[LibraryEntry] | None = None,
+        *,
+        enabled: bool = True,
+        error: Exception | None = None,
+        token: object | None = None,
+    ) -> None:
+        self.name = name
+        self.display_name = name.title()
+        self.content_types = frozenset(content_types)
+        self.entries = entries or []
+        self.enabled = enabled
+        self.error = error
+        self.token = token
+        self.fetches = 0
+
+    def is_enabled(self) -> bool:
+        return self.enabled
+
+    def describe(self) -> str:
+        return f"{self.display_name} (test)"
+
+    def fingerprint(self) -> object | None:
+        return self.token
+
+    def fetch_entries(self) -> list[LibraryEntry]:
+        self.fetches += 1
+        if self.error is not None:
+            raise self.error
+        return list(self.entries)
+
+
+@pytest.fixture
+def providers(monkeypatch: pytest.MonkeyPatch) -> list[_Provider]:
+    registered: list[_Provider] = []
+    monkeypatch.setattr(library_index, "all_providers", lambda overrides=None: list(registered))
+    monkeypatch.setattr(library_index, "_cache", {})
+    return registered
+
+
+@pytest.fixture
+def clock(monkeypatch: pytest.MonkeyPatch) -> list[float]:
+    now = [1000.0]
+    monkeypatch.setattr(library_index.time, "monotonic", lambda: now[0])
+    return now
+
+
+def _warnings(monkeypatch: pytest.MonkeyPatch) -> list[str]:
+    messages: list[str] = []
+    monkeypatch.setattr(
+        library_index.logger, "warning", lambda msg, *args: messages.append(msg % args)
+    )
+    return messages
+
+
+# --------------------------------------------------------------------------- #
+# ISBN forms
+# --------------------------------------------------------------------------- #
+def test_isbn_variants_add_the_isbn13_form_of_an_isbn10() -> None:
+    assert isbn_variants("0-593-82024-X") == {"059382024X", "9780593820247"}
+    assert isbn_variants("978-0-593-82024-7") == {"9780593820247"}
+    assert isbn_variants(None) == frozenset()
+    assert isbn_variants("n/a") == frozenset()
+
+
+# --------------------------------------------------------------------------- #
+# Pure matcher
+# --------------------------------------------------------------------------- #
+def test_no_entries_never_match() -> None:
+    assert library_index.book_matches_entries(_book(), []) is False
+
+
+def test_isbn13_exact_match() -> None:
+    entries = [_entry("unrelated", isbns=frozenset({"9780593820247"}))]
+    book = _book(title="Different Title", authors=["Someone Else"], isbn_13="978-0-593-82024-7")
+
+    assert library_index.book_matches_entries(book, entries) is True
+
+
+def test_isbn10_book_matches_isbn13_entry() -> None:
+    entries = [_entry("unrelated", isbns=frozenset({"9780593820247"}))]
+    book = _book(title="Different Title", authors=["Someone Else"], isbn_10="059382024X")
+
+    assert library_index.book_matches_entries(book, entries) is True
+
+
+def test_isbn13_book_matches_entry_indexed_from_an_isbn10() -> None:
+    entries = [_entry("unrelated", isbns=isbn_variants("059382024X"))]
+    book = _book(title="Different Title", authors=["Someone Else"], isbn_13="9780593820247")
+
+    assert library_index.book_matches_entries(book, entries) is True
+
+
+def test_external_id_match_requires_the_same_provider() -> None:
+    entries = [_entry("unrelated", external_ids=frozenset({("hardcover", "446681")}))]
+    book = _book(title="Different Title", authors=["Someone Else"])
+
+    assert library_index.book_matches_entries(book, entries) is True
+    assert library_index.book_matches_entries(book, [_entry("x")]) is False
+    other_provider = _book(provider="googlebooks", title="Different", authors=["Else"])
+    assert library_index.book_matches_entries(other_provider, entries) is False
+
+
+def test_fuzzy_title_and_author_surname_match() -> None:
+    assert library_index.book_matches_entries(_book(), [_DCC_ENTRY]) is True
+
+
+def test_near_miss_title_fails() -> None:
+    sequel = _book(title="Dungeon Crawler Carl 2: Carl's Doomsday Scenario")
+
+    assert library_index.book_matches_entries(sequel, [_DCC_ENTRY]) is False
+
+
+def test_right_title_wrong_author_fails() -> None:
+    assert library_index.book_matches_entries(_book(authors=["Andy Weir"]), [_DCC_ENTRY]) is False
+
+
+def test_search_fields_take_precedence_over_display_fields() -> None:
+    book = _book(
+        title="Dungeon Crawler Carl: A LitRPG Adventure", search_title="Dungeon Crawler Carl"
+    )
+
+    assert library_index.book_matches_entries(book, [_DCC_ENTRY]) is True
+
+
+# --------------------------------------------------------------------------- #
+# Routing
+# --------------------------------------------------------------------------- #
+def test_ebook_request_consults_only_the_ebook_provider(providers: list[_Provider]) -> None:
+    audio = _Provider("audio", {"audiobook"}, [_DCC_ENTRY])
+    ebook = _Provider("ebook", {"ebook"}, [])
+    providers.extend([audio, ebook])
+
+    assert library_index.is_in_library(_book(), "ebook") is False
+    assert (audio.fetches, ebook.fetches) == (0, 1)
+
+    assert library_index.is_in_library(_book(), "audiobook") is True
+    assert (audio.fetches, ebook.fetches) == (1, 1)
+
+
+def test_no_content_type_consults_every_enabled_provider(providers: list[_Provider]) -> None:
+    audio = _Provider("audio", {"audiobook"}, [])
+    ebook = _Provider("ebook", {"ebook"}, [_DCC_ENTRY])
+    providers.extend([audio, ebook])
+
+    assert library_index.is_in_library(_book()) is True
+    assert (audio.fetches, ebook.fetches) == (1, 1)
+
+
+def test_disabled_provider_is_not_consulted(providers: list[_Provider]) -> None:
+    disabled = _Provider("ebook", {"ebook"}, [_DCC_ENTRY], enabled=False)
+    providers.append(disabled)
+
+    assert library_index.is_in_library(_book(), "ebook") is False
+    assert disabled.fetches == 0
+
+
+def test_any_provider_enabled(providers: list[_Provider]) -> None:
+    assert library_index.any_provider_enabled() is False
+
+    providers.append(_Provider("ebook", {"ebook"}, enabled=False))
+    assert library_index.any_provider_enabled() is False
+
+    providers.append(_Provider("audio", {"audiobook"}))
+    assert library_index.any_provider_enabled() is True
+
+
+# --------------------------------------------------------------------------- #
+# Fail-open + cache
+# --------------------------------------------------------------------------- #
+def test_provider_error_fails_open_without_a_cache(
+    providers: list[_Provider], monkeypatch: pytest.MonkeyPatch
+) -> None:
+    providers.append(_Provider("ebook", {"ebook"}, error=OSError("no such file")))
+    warnings = _warnings(monkeypatch)
+
+    assert library_index.is_in_library(_book(), "ebook") is False
+    assert warnings == ["library check: Ebook (test) unavailable (no such file); failing open"]
+
+
+def test_provider_error_keeps_answering_from_the_stale_cache(
+    providers: list[_Provider], monkeypatch: pytest.MonkeyPatch, clock: list[float]
+) -> None:
+    provider = _Provider("ebook", {"ebook"}, [_DCC_ENTRY])
+    providers.append(provider)
+    assert library_index.is_in_library(_book(), "ebook") is True
+
+    provider.error = RuntimeError("boom")
+    clock[0] += library_index._CACHE_TTL_SECONDS + 1
+    warnings = _warnings(monkeypatch)
+
+    assert library_index.is_in_library(_book(), "ebook") is True
+    assert provider.fetches == 2
+    assert len(warnings) == 1
+
+
+def test_entries_are_cached_until_the_ttl_expires(
+    providers: list[_Provider], clock: list[float]
+) -> None:
+    provider = _Provider("ebook", {"ebook"}, [_DCC_ENTRY])
+    providers.append(provider)
+
+    library_index.is_in_library(_book(), "ebook")
+    library_index.is_in_library(_book(), "ebook")
+    assert provider.fetches == 1
+
+    clock[0] += library_index._CACHE_TTL_SECONDS - 1
+    library_index.is_in_library(_book(), "ebook")
+    assert provider.fetches == 1
+
+    clock[0] += 2
+    library_index.is_in_library(_book(), "ebook")
+    assert provider.fetches == 2
+
+
+def test_fingerprint_change_refreshes_before_the_ttl(
+    providers: list[_Provider], clock: list[float]
+) -> None:
+    provider = _Provider("ebook", {"ebook"}, [], token=1.0)
+    providers.append(provider)
+
+    assert library_index.is_in_library(_book(), "ebook") is False
+    clock[0] += 5
+    library_index.is_in_library(_book(), "ebook")
+    assert provider.fetches == 1
+
+    provider.entries = [_DCC_ENTRY]
+    provider.token = 2.0
+    assert library_index.is_in_library(_book(), "ebook") is True
+    assert provider.fetches == 2
+
+
+def test_cache_is_kept_per_provider(providers: list[_Provider]) -> None:
+    audio = _Provider("audio", {"audiobook"}, [])
+    ebook = _Provider("ebook", {"ebook"}, [_DCC_ENTRY])
+    providers.extend([audio, ebook])
+
+    assert library_index.is_in_library(_book(), "audiobook") is False
+    assert library_index.is_in_library(_book(), "ebook") is True
+    assert library_index.is_in_library(_book(), "audiobook") is False
+    assert (audio.fetches, ebook.fetches) == (1, 1)
+
+
+# --------------------------------------------------------------------------- #
+# Settings test button
+# --------------------------------------------------------------------------- #
+def test_connection_reports_the_count_and_primes_the_cache(providers: list[_Provider]) -> None:
+    provider = _Provider("ebook", {"ebook"}, [_DCC_ENTRY, _entry("other")])
+    providers.append(provider)
+
+    result = library_index.test_connection("ebook", None)
+
+    assert result == {"success": True, "message": "Ebook (test): indexed 2 item(s)."}
+    assert library_index.is_in_library(_book(), "ebook") is True
+    assert provider.fetches == 1
+
+
+def test_connection_reports_provider_errors(providers: list[_Provider]) -> None:
+    providers.append(_Provider("ebook", {"ebook"}, error=OSError("no such file")))
+
+    result = library_index.test_connection("ebook", None)
+
+    assert result == {"success": False, "message": "Ebook (test): no such file"}
+
+
+def test_connection_rejects_unknown_providers(providers: list[_Provider]) -> None:
+    result = library_index.test_connection("nope", None)
+
+    assert result == {"success": False, "message": "Unknown library provider: nope"}
+
+
+def test_unsaved_form_values_override_saved_settings_for_test_connection(tmp_path):
+    from shelfmark.core.library_providers import setting
+    from shelfmark.core.library_providers.calibre import CalibreLibrary
+
+    class _Saved:
+        def get(self, key, default=None, user_id=None):
+            return {"CALIBRE_LIBRARY_DB_PATH": "/saved/metadata.db"}.get(key, default)
+
+    assert (
+        setting(_Saved(), "CALIBRE_LIBRARY_DB_PATH", "", {"CALIBRE_LIBRARY_DB_PATH": "/form/x.db"})
+        == "/form/x.db"
+    )
+    assert (
+        setting(_Saved(), "CALIBRE_LIBRARY_DB_PATH", "", {"CALIBRE_LIBRARY_DB_PATH": ""})
+        == "/saved/metadata.db"
+    )
+    assert setting(_Saved(), "CALIBRE_LIBRARY_DB_PATH", "", None) == "/saved/metadata.db"
+
+    provider = CalibreLibrary({"CALIBRE_LIBRARY_DB_PATH": str(tmp_path / "form.db")})
+    assert provider.describe().endswith("form.db")
+
+
+def test_ownership_reports_only_formats_with_an_enabled_library(providers):
+    owned = LibraryEntry(
+        frozenset({"dungeon", "crawler", "carl"}),
+        frozenset({"9780593820247"}),
+        frozenset(),
+        frozenset(),
+    )
+    providers.append(_Provider("calibre", {"ebook"}, [owned]))
+    providers.append(_Provider("audiobooks", {"audiobook"}, [], enabled=False))
+
+    assert library_index.ownership(_book(isbn_13="9780593820247")) == {"ebook": "owned"}
+    assert library_index.ownership(_book(isbn_13="9999999999999", title="Something Else")) == {
+        "ebook": None
+    }
+
+
+def test_ownership_is_none_when_no_library_check_is_enabled(providers):
+    providers.append(_Provider("calibre", {"ebook"}, [], enabled=False))
+
+    assert library_index.ownership(_book()) is None
+
+
+def test_ownership_covers_both_formats_when_both_libraries_are_enabled(providers):
+    entry = LibraryEntry(
+        frozenset({"dungeon", "crawler", "carl"}),
+        frozenset({"9780593820247"}),
+        frozenset(),
+        frozenset(),
+    )
+    providers.append(_Provider("calibre", {"ebook"}, []))
+    providers.append(_Provider("audiobooks", {"audiobook"}, [entry]))
+
+    assert library_index.ownership(_book(isbn_13="9780593820247")) == {
+        "ebook": None,
+        "audiobook": "owned",
+    }
+
+
+def _shelf_entry(title_tokens: set[str], context: set[str] | None = None) -> LibraryEntry:
+    context = context or set()
+    return LibraryEntry(
+        frozenset(title_tokens | context),
+        frozenset(),
+        frozenset(),
+        frozenset(),
+        frozenset(title_tokens),
+        frozenset(context),
+    )
+
+
+def test_a_longer_shelf_title_naming_another_work_is_not_a_match() -> None:
+    # "Dune" is a subset of "Dune Messiah", which is a different book.
+    shelf = [_shelf_entry({"dune", "messiah"}, {"frank", "herbert"})]
+    book = _book(title="Dune", authors=["Frank Herbert"])
+
+    assert library_index.match_entries(book, shelf) is None
+
+
+def test_the_series_and_volume_a_shelf_title_repeats_do_not_block_a_match() -> None:
+    # Calibre titles often carry the series and its number: "Alex Cross 25: Cross Kill".
+    shelf = [_shelf_entry({"alex", "cross", "25", "kill"}, {"alex", "cross", "james", "patterson"})]
+    book = _book(title="Cross Kill", authors=["James Patterson"])
+
+    assert library_index.match_entries(book, shelf) == "owned"
+
+
+def test_an_edition_word_does_not_block_a_match() -> None:
+    shelf = [_shelf_entry({"dune", "illustrated", "edition"}, {"frank", "herbert"})]
+
+    assert (
+        library_index.match_entries(_book(title="Dune", authors=["Frank Herbert"]), shelf)
+        == "owned"
+    )
+
+
+def test_a_collection_reports_itself_as_a_collection() -> None:
+    shelf = [_shelf_entry({"dungeon", "crawler", "carl", "books", "1", "6"}, {"matt", "dinniman"})]
+    book = _book(title="Dungeon Crawler Carl", authors=["Matt Dinniman"])
+
+    assert library_index.match_entries(book, shelf) == "collection"
+
+
+def test_an_exact_shelf_title_outranks_a_collection_holding_the_same_book() -> None:
+    collection = _shelf_entry({"dungeon", "crawler", "carl", "omnibus"}, {"matt", "dinniman"})
+    single = _shelf_entry({"dungeon", "crawler", "carl"}, {"matt", "dinniman"})
+    book = _book(title="Dungeon Crawler Carl", authors=["Matt Dinniman"])
+
+    assert library_index.match_entries(book, [collection, single]) == "owned"
diff --git a/tests/core/test_library_provider_calibre.py b/tests/core/test_library_provider_calibre.py
new file mode 100644
index 00000000..6894c84f
--- /dev/null
+++ b/tests/core/test_library_provider_calibre.py
@@ -0,0 +1,318 @@
+"""Tests for the read-only Calibre metadata.db library provider."""
+
+from __future__ import annotations
+
+import os
+import sqlite3
+from pathlib import Path
+from typing import Any
+
+import pytest
+
+from shelfmark.core import library_index
+from shelfmark.core.library_providers import calibre
+from shelfmark.metadata_providers import BookMetadata
+
+_SCHEMA = """
+CREATE TABLE books (id INTEGER PRIMARY KEY, title TEXT NOT NULL, uuid TEXT);
+CREATE TABLE authors (id INTEGER PRIMARY KEY, name TEXT NOT NULL);
+CREATE TABLE books_authors_link (id INTEGER PRIMARY KEY, book INTEGER NOT NULL, author INTEGER NOT NULL);
+CREATE TABLE series (id INTEGER PRIMARY KEY, name TEXT NOT NULL);
+CREATE TABLE books_series_link (id INTEGER PRIMARY KEY, book INTEGER NOT NULL, series INTEGER NOT NULL);
+CREATE TABLE identifiers (id INTEGER PRIMARY KEY, book INTEGER NOT NULL, type TEXT NOT NULL, val TEXT NOT NULL);
+"""
+
+_BOOKS = [
+    (1, "Dungeon Crawler Carl", "u1"),
+    (2, "Good Omens", "u2"),
+    (3, "Untitled Draft", "u3"),
+    (4, "The Martian", "u4"),
+]
+_AUTHORS = [(1, "Matt Dinniman"), (2, "Terry Pratchett"), (3, "Neil Gaiman"), (4, "Andy Weir")]
+_AUTHOR_LINKS = [(1, 1), (2, 2), (2, 3), (4, 4)]  # book 3 has no author
+_SERIES = [(1, "Dungeon Crawler Carl")]
+_SERIES_LINKS = [(1, 1)]
+_IDENTIFIERS = [
+    (1, "isbn", "9780593820247"),
+    (1, "mobi-asin", "b08bkgyqxw"),
+    (1, "hardcover-id", "446681"),
+    (1, "google", "yEem0QEACAAJ"),
+    (2, "isbn-10", "0060853980"),
+    (2, "amazon", "B000FC0V3W"),
+    (2, "goodreads", "12067"),
+    (4, "isbn-13", "978-0-8041-3902-1"),
+    (4, "openlibrary", "OL17091839W"),
+]
+
+
+def _make_library(path: Path, *, wal: bool = False) -> None:
+    conn = sqlite3.connect(path)
+    if wal:
+        conn.execute("PRAGMA journal_mode=wal")
+    conn.executescript(_SCHEMA)
+    conn.executemany("INSERT INTO books(id, title, uuid) VALUES (?, ?, ?)", _BOOKS)
+    conn.executemany("INSERT INTO authors(id, name) VALUES (?, ?)", _AUTHORS)
+    conn.executemany("INSERT INTO books_authors_link(book, author) VALUES (?, ?)", _AUTHOR_LINKS)
+    conn.executemany("INSERT INTO series(id, name) VALUES (?, ?)", _SERIES)
+    conn.executemany("INSERT INTO books_series_link(book, series) VALUES (?, ?)", _SERIES_LINKS)
+    conn.executemany("INSERT INTO identifiers(book, type, val) VALUES (?, ?, ?)", _IDENTIFIERS)
+    conn.commit()
+    conn.close()
+
+
+def _configure(monkeypatch: pytest.MonkeyPatch, path: Path, *, enabled: bool = True) -> None:
+    values: dict[str, Any] = {
+        "CALIBRE_LIBRARY_DB_PATH": str(path),
+        "LIBRARY_CHECK_CALIBRE_ENABLED": enabled,
+    }
+    monkeypatch.setattr(
+        calibre.app_config,
+        "get",
+        lambda key, default=None, user_id=None: values.get(key, default),
+    )
+    monkeypatch.setattr(library_index, "_cache", {})
+
+
+def _infos(monkeypatch: pytest.MonkeyPatch) -> list[str]:
+    messages: list[str] = []
+    monkeypatch.setattr(calibre.logger, "info", lambda msg, *args: messages.append(msg % args))
+    return messages
+
+
+def _by_token(entries: list[Any], token: str) -> Any:
+    matches = [entry for entry in entries if token in entry.tokens]
+    assert len(matches) == 1, token
+    return matches[0]
+
+
+def _dcc() -> BookMetadata:
+    return BookMetadata(
+        provider="hardcover",
+        provider_id="446681",
+        title="Dungeon Crawler Carl",
+        authors=["Matt Dinniman"],
+    )
+
+
+def test_indexes_titles_authors_series_and_identifiers(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    path = tmp_path / "metadata.db"
+    _make_library(path)
+    _configure(monkeypatch, path)
+
+    entries = calibre.CalibreLibrary().fetch_entries()
+
+    assert len(entries) == 4
+
+    dcc = _by_token(entries, "dinniman")
+    assert {"dungeon", "crawler", "carl", "matt"} <= dcc.tokens
+    assert dcc.isbns == {"9780593820247"}
+    assert dcc.asins == {"B08BKGYQXW"}
+    assert dcc.external_ids == {("hardcover", "446681"), ("googlebooks", "yEem0QEACAAJ")}
+
+    omens = _by_token(entries, "omens")
+    assert {"pratchett", "gaiman"} <= omens.tokens
+    assert omens.isbns == {"0060853980", "9780060853983"}
+    assert omens.asins == {"B000FC0V3W"}
+    assert omens.external_ids == frozenset()
+
+    martian = _by_token(entries, "martian")
+    assert martian.isbns == {"9780804139021"}
+    assert martian.external_ids == {("openlibrary", "OL17091839W")}
+
+    draft = _by_token(entries, "draft")
+    assert draft.tokens == {"untitled", "draft"}
+    assert (draft.isbns, draft.asins, draft.external_ids) == (frozenset(), frozenset(), frozenset())
+
+
+def test_reads_a_read_only_file(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> None:
+    path = tmp_path / "metadata.db"
+    _make_library(path)
+    path.chmod(0o444)
+    _configure(monkeypatch, path)
+
+    assert len(calibre.CalibreLibrary().fetch_entries()) == 4
+
+
+def test_wal_reads_see_uncheckpointed_writes(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    path = tmp_path / "metadata.db"
+    _make_library(path, wal=True)
+    _configure(monkeypatch, path)
+    infos = _infos(monkeypatch)
+
+    writer = sqlite3.connect(path)
+    writer.execute("PRAGMA wal_autocheckpoint=0")
+    writer.execute("INSERT INTO books(id, title, uuid) VALUES (5, 'Fresh Arrival', 'u5')")
+    writer.commit()
+    try:
+        assert (tmp_path / "metadata.db-wal").exists()
+        entries = calibre.CalibreLibrary().fetch_entries()
+    finally:
+        writer.close()
+
+    assert len(entries) == 5
+    assert infos == []
+
+
+@pytest.mark.skipif(os.geteuid() == 0, reason="root is not bound by directory permissions")
+def test_wal_without_side_files_on_a_read_only_mount_uses_a_snapshot(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    lib_dir = tmp_path / "library"
+    lib_dir.mkdir()
+    path = lib_dir / "metadata.db"
+    _make_library(path, wal=True)
+    for suffix in ("-wal", "-shm"):
+        (lib_dir / f"metadata.db{suffix}").unlink(missing_ok=True)
+    _configure(monkeypatch, path)
+    infos = _infos(monkeypatch)
+
+    lib_dir.chmod(0o555)
+    try:
+        entries = calibre.CalibreLibrary().fetch_entries()
+    finally:
+        lib_dir.chmod(0o755)
+
+    assert len(entries) == 4
+    assert not (lib_dir / "metadata.db-wal").exists()
+    assert len(infos) == 1
+    assert "immutable snapshot" in infos[0]
+
+
+def test_in_place_open_failure_falls_back_to_an_immutable_open(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    path = tmp_path / "metadata.db"
+    _make_library(path)
+    _configure(monkeypatch, path)
+    infos = _infos(monkeypatch)
+
+    real_connect = sqlite3.connect
+    uris: list[str] = []
+
+    def fake_connect(database: str, **kwargs: Any) -> sqlite3.Connection:
+        uris.append(database)
+        if "immutable=1" not in database:
+            raise sqlite3.OperationalError("attempt to write a readonly database")
+        return real_connect(database, **kwargs)
+
+    monkeypatch.setattr(calibre.sqlite3, "connect", fake_connect)
+
+    assert len(calibre.CalibreLibrary().fetch_entries()) == 4
+    assert [uri.endswith("?mode=ro") for uri in uris] == [True, False]
+    assert uris[1].endswith("?mode=ro&immutable=1")
+    assert "attempt to write a readonly database" in infos[0]
+
+
+def test_locked_database_falls_back_to_a_snapshot(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    path = tmp_path / "metadata.db"
+    _make_library(path)
+    _configure(monkeypatch, path)
+    monkeypatch.setattr(calibre, "_BUSY_TIMEOUT_SECONDS", 0.01)
+    infos = _infos(monkeypatch)
+
+    writer = sqlite3.connect(path, isolation_level=None)
+    writer.execute("BEGIN EXCLUSIVE")
+    writer.execute("INSERT INTO books(id, title, uuid) VALUES (5, 'Pending', 'u5')")
+    try:
+        entries = calibre.CalibreLibrary().fetch_entries()
+    finally:
+        writer.rollback()
+        writer.close()
+
+    assert len(entries) == 4
+    assert "database is locked" in infos[0]
+
+
+def test_unreadable_database_raises_and_the_facade_fails_open(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    path = tmp_path / "metadata.db"
+    _make_library(path)
+    _configure(monkeypatch, path)
+
+    def always_fail(database: str, **kwargs: Any) -> sqlite3.Connection:
+        raise sqlite3.OperationalError("disk I/O error")
+
+    monkeypatch.setattr(calibre.sqlite3, "connect", always_fail)
+    warnings: list[str] = []
+    monkeypatch.setattr(
+        library_index.logger, "warning", lambda msg, *args: warnings.append(msg % args)
+    )
+
+    with pytest.raises(sqlite3.OperationalError):
+        calibre.CalibreLibrary().fetch_entries()
+    assert library_index.is_in_library(_dcc(), "ebook") is False
+    assert warnings == [
+        f"library check: Calibre library at {path} unavailable (disk I/O error); failing open"
+    ]
+
+
+def test_missing_file_is_enabled_but_fetch_raises_cleanly(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    path = tmp_path / "missing.db"
+    _configure(monkeypatch, path)
+    provider = calibre.CalibreLibrary()
+
+    assert provider.is_enabled() is True
+    assert provider.describe() == f"Calibre library at {path}"
+    assert provider.fingerprint() is None
+    with pytest.raises(FileNotFoundError):
+        provider.fetch_entries()
+
+    result = library_index.test_connection("calibre", None)
+    assert result["success"] is False
+    assert str(path) in result["message"]
+    assert library_index.is_in_library(_dcc(), "ebook") is False
+
+
+def test_disabled_provider_is_not_consulted(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    path = tmp_path / "metadata.db"
+    _make_library(path)
+    _configure(monkeypatch, path, enabled=False)
+
+    assert calibre.CalibreLibrary().is_enabled() is False
+    assert library_index.any_provider_enabled() is False
+    assert library_index.is_in_library(_dcc(), "ebook") is False
+
+
+def test_enabled_provider_answers_ebook_lookups_only(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    path = tmp_path / "metadata.db"
+    _make_library(path)
+    _configure(monkeypatch, path)
+
+    assert library_index.any_provider_enabled() is True
+    assert library_index.is_in_library(_dcc(), "ebook") is True
+    assert library_index.is_in_library(_dcc(), "audiobook") is False
+    assert library_index.test_connection("calibre", None) == {
+        "success": True,
+        "message": f"Calibre library at {path}: indexed 4 item(s).",
+    }
+
+
+def test_fingerprint_follows_the_database_and_wal_mtime(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    path = tmp_path / "metadata.db"
+    _make_library(path)
+    _configure(monkeypatch, path)
+    provider = calibre.CalibreLibrary()
+
+    assert provider.fingerprint() == path.stat().st_mtime
+
+    wal = tmp_path / "metadata.db-wal"
+    wal.write_bytes(b"")
+    later = path.stat().st_mtime + 60
+    os.utime(wal, (later, later))
+
+    assert provider.fingerprint() == later
diff --git a/tests/core/test_metadata_search_library.py b/tests/core/test_metadata_search_library.py
new file mode 100644
index 00000000..17bba7f4
--- /dev/null
+++ b/tests/core/test_metadata_search_library.py
@@ -0,0 +1,71 @@
+"""The metadata search and book endpoints carry per-format library ownership."""
+
+from __future__ import annotations
+
+import importlib
+from unittest.mock import patch
+
+import pytest
+
+from shelfmark.metadata_providers import BookMetadata, SearchResult
+
+
+@pytest.fixture(scope="module")
+def main_module():
+    with patch("shelfmark.download.orchestrator.start"):
+        import shelfmark.main as main
+
+        importlib.reload(main)
+        return main
+
+
+@pytest.fixture
+def client(main_module):
+    return main_module.app.test_client()
+
+
+class _StubProvider:
+    name = "stub"
+    display_name = "Stub"
+    search_fields: list[object] = []
+
+    def __init__(self, books: list[BookMetadata]) -> None:
+        self._books = books
+
+    def is_available(self) -> bool:
+        return True
+
+    def search_paginated(self, options):
+        return SearchResult(books=self._books, page=1, total_found=len(self._books), has_more=False)
+
+    def get_book(self, book_id: str) -> BookMetadata | None:
+        return next((b for b in self._books if b.provider_id == book_id), None)
+
+
+def _search(main_module, client, ownership):
+    book = BookMetadata(provider="stub", provider_id="42", title="Dungeon Crawler Carl")
+    stub = _StubProvider([book])
+    with (
+        patch.object(main_module, "get_auth_mode", return_value="none"),
+        patch("shelfmark.metadata_providers.is_provider_registered", return_value=True),
+        patch("shelfmark.metadata_providers.is_provider_enabled", return_value=True),
+        patch("shelfmark.metadata_providers.get_provider_kwargs", return_value={}),
+        patch("shelfmark.metadata_providers.get_provider", return_value=stub),
+        patch("shelfmark.metadata_providers.get_configured_provider", return_value=stub),
+        patch("shelfmark.core.library_index.ownership", ownership),
+    ):
+        response = client.get("/api/metadata/search?query=carl&provider=stub")
+    assert response.status_code == 200, response.get_json()
+    return response.get_json()["books"][0]
+
+
+def test_search_results_carry_library_ownership(main_module, client):
+    result = _search(main_module, client, lambda book: {"ebook": True, "audiobook": False})
+
+    assert result["library"] == {"ebook": True, "audiobook": False}
+
+
+def test_search_results_omit_library_when_no_check_is_enabled(main_module, client):
+    result = _search(main_module, client, lambda book: None)
+
+    assert "library" not in result
diff --git a/tests/core/test_text_match.py b/tests/core/test_text_match.py
new file mode 100644
index 00000000..cf2fcd34
--- /dev/null
+++ b/tests/core/test_text_match.py
@@ -0,0 +1,77 @@
+"""Tests for the shared token/ISBN/surname matching helpers."""
+
+import pytest
+
+from shelfmark.core import text_match
+
+_TWENTY_WORDS = " ".join(f"word{i:02d}" for i in range(20))
+
+
+def test_tokens_lowercases_and_splits_on_non_alphanumerics():
+    assert text_match.tokens("Dungeon Crawler Carl: Book 1!") == [
+        "dungeon",
+        "crawler",
+        "carl",
+        "book",
+        "1",
+    ]
+
+
+@pytest.mark.parametrize("value", [None, "", "  ---  "])
+def test_tokens_empty_input(value):
+    assert text_match.tokens(value) == []
+
+
+def test_significant_tokens_drops_stopwords_and_single_characters():
+    assert text_match.significant_tokens("The Way of Kings, Part A 2") == ["way", "kings", "part"]
+
+
+@pytest.mark.parametrize(
+    ("author", "expected"),
+    [
+        ("Sanderson, Brandon", "sanderson"),
+        ("Brandon Sanderson", "sanderson"),
+        ("Dinniman, Matt J.", "dinniman"),
+        ("", None),
+        (None, None),
+        ("A", None),
+    ],
+)
+def test_author_surname(author, expected):
+    assert text_match.author_surname(author) == expected
+
+
+def test_title_tokens_match_default_threshold_edges():
+    words = _TWENTY_WORDS.split()
+
+    assert text_match.title_tokens_match(_TWENTY_WORDS, set(words[:17])) is True  # 17/20 == 0.85
+    assert text_match.title_tokens_match(_TWENTY_WORDS, set(words[:16])) is False
+
+
+def test_title_tokens_match_explicit_threshold_is_inclusive():
+    assert text_match.title_tokens_match("alpha beta", {"alpha"}, threshold=0.5) is True
+    assert text_match.title_tokens_match("alpha beta", {"alpha"}, threshold=0.51) is False
+
+
+def test_title_tokens_match_ignores_stopwords_in_title():
+    assert text_match.title_tokens_match("The Name of the Wind", {"name", "wind"}) is True
+
+
+@pytest.mark.parametrize("title", [None, "", "the of a"])
+def test_title_tokens_match_without_significant_tokens_is_false(title):
+    assert text_match.title_tokens_match(title, {"the", "of"}) is False
+
+
+@pytest.mark.parametrize(
+    ("value", "expected"),
+    [
+        ("978-0-59-382024-7", "9780593820247"),
+        ("0-306-40615-x", "030640615X"),
+        (" 0306406152 ", "0306406152"),
+        (9780593820247, "9780593820247"),
+        (None, ""),
+        ("", ""),
+    ],
+)
+def test_normalize_isbn(value, expected):
+    assert text_match.normalize_isbn(value) == expected