diff --git a/shelfmark/config/settings.py b/shelfmark/config/settings.py
index f26086d1..04d526c2 100644
--- a/shelfmark/config/settings.py
+++ b/shelfmark/config/settings.py
@@ -878,15 +878,13 @@ def _on_save_downloads(values: dict[str, Any]) -> dict[str, Any]:
     if books_output_mode == "email":
         from email.utils import parseaddr
 
-        def _is_plain_email_address(addr: str) -> bool:
-            parsed = parseaddr(addr or "")[1]
-            return bool(parsed) and "@" in parsed and parsed == addr
+        from shelfmark.core.request_helpers import is_plain_email_address
 
         # Preferred model: single recipient for global default and per-user override.
         raw_recipient = str(effective.get("EMAIL_RECIPIENT", "") or "").strip()
 
         # Optional global fallback: validate only when a default recipient is provided.
-        if raw_recipient and not _is_plain_email_address(raw_recipient):
+        if raw_recipient and not is_plain_email_address(raw_recipient):
             return {
                 "error": True,
                 "message": "Email recipient must be a valid plain email address.",
diff --git a/shelfmark/core/admin_settings_routes.py b/shelfmark/core/admin_settings_routes.py
index 18b91f88..f02c92fb 100644
--- a/shelfmark/core/admin_settings_routes.py
+++ b/shelfmark/core/admin_settings_routes.py
@@ -16,8 +16,14 @@ from shelfmark.config.users_settings import (
     validate_search_preference_value,
 )
 from shelfmark.core.config import config as app_config
+from shelfmark.core.request_helpers import coerce_int, is_plain_email_address
 from shelfmark.core.request_policy import parse_policy_mode, validate_policy_rules
-from shelfmark.core.settings_registry import load_config_file
+from shelfmark.core.settings_registry import (
+    FieldBase,
+    SelectField,
+    TextField,
+    load_config_file,
+)
 from shelfmark.core.user_settings_overrides import (
     build_user_preferences_payload as _build_user_preferences_payload,
 )
@@ -27,36 +33,117 @@ from shelfmark.core.user_settings_overrides import (
 from shelfmark.core.user_settings_overrides import (
     get_settings_registry as _get_settings_registry,
 )
+from shelfmark.core.utils import get_global_destination, resolve_user_destination_override
 
 if TYPE_CHECKING:
-    from collections.abc import Callable
+    from collections.abc import Callable, Mapping
 
     from flask.typing import ResponseReturnValue
 
     from shelfmark.core.user_db import UserDB
 
 
+_USER_DESTINATION_KEYS = {"DESTINATION", "DESTINATION_AUDIOBOOK"}
+_BOOKLORE_ID_KEYS = {"BOOKLORE_LIBRARY_ID", "BOOKLORE_PATH_ID"}
+
+
+def _validate_self_service_value(
+    key: str,
+    value: Any,
+    field: FieldBase,
+    user: Mapping[str, Any],
+) -> tuple[Any, str | None]:
+    """Check a value a user saves for themselves; admins are not limited this way.
+
+    Returns (normalized_value, error). The value is returned unchanged when no
+    rule applies to the field.
+    """
+    if key in _USER_DESTINATION_KEYS:
+        if not isinstance(value, str):
+            return None, f"Invalid value for {key}: must be a string"
+        if not value.strip():
+            return value, None
+        base = get_global_destination(is_audiobook=key == "DESTINATION_AUDIOBOOK")
+        raw_user_id = user.get("id")
+        resolved = resolve_user_destination_override(
+            value,
+            base,
+            user_id=raw_user_id if isinstance(raw_user_id, int) else None,
+            username=str(user.get("username") or ""),
+        )
+        if resolved is None:
+            return None, f"Invalid value for {key}: must be a folder inside {base}"
+        return value, None
+
+    if key in _BOOKLORE_ID_KEYS:
+        parsed = coerce_int(value, -1)
+        if parsed < 0 or str(parsed) != str(value).strip():
+            return None, f"Invalid value for {key}: must be a non-negative integer"
+        return value, None
+
+    if key == "EMAIL_RECIPIENT":
+        if not isinstance(value, str):
+            return None, f"Invalid value for {key}: must be a string"
+        recipient = value.strip()
+        if recipient and not is_plain_email_address(recipient):
+            return None, f"Invalid value for {key}: must be a plain email address"
+        return recipient, None
+
+    # Search preferences have their own validator (case-insensitive values).
+    if (
+        isinstance(field, SelectField)
+        and key not in SEARCH_PREFERENCE_VALIDATABLE_KEYS
+        and not callable(field.options)
+    ):
+        options = field.options if isinstance(field.options, list) else []
+        allowed = {option.get("value") for option in options if isinstance(option, dict)}
+        if not isinstance(value, str) or value not in allowed:
+            return None, f"Invalid value for {key}: {value!r} is not one of the options"
+        return value, None
+
+    if isinstance(field, TextField) and not isinstance(value, str):
+        return None, f"Invalid value for {key}: must be a string"
+
+    return value, None
+
+
 def validate_user_settings(
     settings: dict[str, Any],
+    *,
+    self_service_user: Mapping[str, Any] | None = None,
 ) -> tuple[dict[str, Any], list[str]]:
-    """Validate and normalize per-user settings overrides."""
+    """Validate and normalize per-user settings overrides.
+
+    ``self_service_user`` is the account saving its own settings; its values are
+    held to the user-scope rules (destinations inside the global one, option
+    lists, plain email). Admin saves on a user's behalf pass None.
+    """
     settings_registry = _get_settings_registry()
     field_map = settings_registry.get_settings_field_map()
     overridable_map = settings_registry.get_user_overridable_fields()
 
     valid: dict[str, Any] = {}
     errors: list[str] = []
-    for key, value in settings.items():
+    for key, raw_value in settings.items():
         if key not in field_map:
             errors.append(f"Unknown setting: {key}")
         elif key not in overridable_map:
             errors.append(f"Setting not user-overridable: {key}")
         else:
             # null means "clear the per-user override; use global default"
-            if value is None:
+            if raw_value is None:
                 valid[key] = None
                 continue
 
+            value = raw_value
+            if self_service_user is not None:
+                value, self_service_error = _validate_self_service_value(
+                    key, raw_value, overridable_map[key][0], self_service_user
+                )
+                if self_service_error:
+                    errors.append(self_service_error)
+                    continue
+
             if (
                 key in {"REQUEST_POLICY_DEFAULT_EBOOK", "REQUEST_POLICY_DEFAULT_AUDIOBOOK"}
                 and parse_policy_mode(value) is None
diff --git a/shelfmark/core/request_helpers.py b/shelfmark/core/request_helpers.py
index 5ac9bfb5..e42ab6d2 100644
--- a/shelfmark/core/request_helpers.py
+++ b/shelfmark/core/request_helpers.py
@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 from datetime import UTC, datetime
+from email.utils import parseaddr
 from typing import Any, Protocol, SupportsIndex, SupportsInt, TypeGuard
 
 from shelfmark.core.config import config as app_config
@@ -115,6 +116,12 @@ def coerce_int(value: object, default: int) -> int:
         return default
 
 
+def is_plain_email_address(value: str) -> bool:
+    """Return True for a bare address like ``reader@example.com`` (no display name)."""
+    parsed = parseaddr(value or "")[1]
+    return bool(parsed) and "@" in parsed and parsed == value
+
+
 def normalize_optional_text(value: object) -> str | None:
     """Return a trimmed string or None for empty/non-string input."""
     if not isinstance(value, str):
diff --git a/shelfmark/core/self_user_routes.py b/shelfmark/core/self_user_routes.py
index 2603a032..2f14a28e 100644
--- a/shelfmark/core/self_user_routes.py
+++ b/shelfmark/core/self_user_routes.py
@@ -454,11 +454,14 @@ def register_self_user_routes(
                     }
                 ), 400
 
-            validated_settings, validation_errors = validate_user_settings(settings_payload)
+            validated_settings, validation_errors = validate_user_settings(
+                settings_payload, self_service_user=user
+            )
             if validation_errors:
                 return jsonify(
                     {
                         "error": "Invalid settings payload",
+                        "message": "; ".join(validation_errors),
                         "details": validation_errors,
                     }
                 ), 400
diff --git a/shelfmark/core/utils.py b/shelfmark/core/utils.py
index 417544cc..e082c3af 100644
--- a/shelfmark/core/utils.py
+++ b/shelfmark/core/utils.py
@@ -10,11 +10,14 @@ from threading import Lock
 from typing import TYPE_CHECKING
 from urllib.parse import urlparse
 
+from shelfmark.core.logger import setup_logger
 from shelfmark.core.request_helpers import normalize_optional_text
 
 if TYPE_CHECKING:
     from types import ModuleType
 
+logger = setup_logger(__name__)
+
 
 def normalize_http_url(
     url: str | None,
@@ -226,38 +229,124 @@ def _expand_user_destination_placeholder(
     return _USER_PLACEHOLDER_PATTERN.sub(resolved_username, path_value)
 
 
+def get_global_destination(*, is_audiobook: bool = False) -> str:
+    """Return the admin-configured destination template, before {User} expansion.
+
+    The audiobook destination falls back to the books destination when unset.
+    """
+    from shelfmark.core.config import config
+
+    if is_audiobook:
+        audiobook_dest = str(config.get("DESTINATION_AUDIOBOOK", "") or "").strip()
+        if audiobook_dest:
+            return audiobook_dest
+    return str(config.get("DESTINATION", "") or config.get("INGEST_DIR", "/books") or "").strip()
+
+
+def resolve_user_destination_override(
+    override: object,
+    base: str,
+    *,
+    user_id: int | None = None,
+    username: str | None = None,
+) -> Path | None:
+    """Resolve a per-user destination against the global one it must stay inside.
+
+    ``{User}`` is expanded in both values. A relative override is taken relative
+    to the global destination. Returns None when the override is empty, contains
+    a ``..`` segment, or does not resolve to the global destination or a path
+    below it; callers then fall back to the global destination.
+    """
+    if not isinstance(override, str) or not override.strip():
+        return None
+
+    base_expanded = _expand_user_destination_placeholder(
+        base.strip(), user_id=user_id, username=username
+    )
+    base_path = Path(os.path.normpath(base_expanded))
+    if not base_path.is_absolute():
+        return None
+
+    candidate = _expand_user_destination_placeholder(
+        override.strip(), user_id=user_id, username=username
+    )
+    if ".." in Path(candidate).parts:
+        return None
+    candidate_path = Path(os.path.normpath(candidate))
+    if not candidate_path.is_absolute():
+        candidate_path = base_path / candidate_path
+    if not candidate_path.is_relative_to(base_path):
+        return None
+    return candidate_path
+
+
+def _get_user_destination_override(
+    key: str,
+    base: str,
+    *,
+    user_id: int,
+    username: str | None,
+) -> Path | None:
+    """Return the user's own destination for ``key`` when it stays inside ``base``."""
+    from shelfmark.core.config import config
+
+    user_value = config.get(key, "", user_id=user_id)
+    if user_value == config.get(key, ""):
+        return None
+    if not isinstance(user_value, str) or not user_value.strip():
+        return None
+
+    resolved = resolve_user_destination_override(
+        user_value, base, user_id=user_id, username=username
+    )
+    if resolved is None:
+        logger.warning(
+            "Ignoring %s override for user %s: %r is outside the configured destination %r",
+            key,
+            user_id,
+            user_value,
+            base,
+        )
+    return resolved
+
+
 def get_destination(
     *,
     is_audiobook: bool = False,
     user_id: int | None = None,
     username: str | None = None,
 ) -> Path:
-    """Get base destination directory. Audiobooks fall back to main destination."""
-    from shelfmark.core.config import config
+    """Get base destination directory. Audiobooks fall back to main destination.
+
+    A per-user override is only honoured while it resolves inside the global
+    destination; anything else falls back to the global value.
+    """
+    books_base = get_global_destination()
 
     if is_audiobook:
-        # Audiobook destination with fallback to main destination
-        audiobook_dest = config.get("DESTINATION_AUDIOBOOK", "", user_id=user_id)
-        if audiobook_dest:
+        audiobook_base = get_global_destination(is_audiobook=True)
+        if user_id is not None:
+            user_audiobook = _get_user_destination_override(
+                "DESTINATION_AUDIOBOOK", audiobook_base, user_id=user_id, username=username
+            )
+            if user_audiobook is not None:
+                return user_audiobook
+        if audiobook_base != books_base:
             return Path(
                 _expand_user_destination_placeholder(
-                    str(audiobook_dest),
-                    user_id=user_id,
-                    username=username,
+                    audiobook_base, user_id=user_id, username=username
                 )
             )
 
-    # Main destination (also fallback for audiobooks)
-    # Check new setting first, then legacy INGEST_DIR
-    destination = config.get("DESTINATION", "", user_id=user_id) or config.get(
-        "INGEST_DIR", "/books"
-    )
-    return Path(
-        _expand_user_destination_placeholder(
-            str(destination),
-            user_id=user_id,
-            username=username,
+    if user_id is not None:
+        user_destination = _get_user_destination_override(
+            "DESTINATION", books_base, user_id=user_id, username=username
         )
+        if user_destination is not None:
+            return user_destination
+
+    return Path(
+        _expand_user_destination_placeholder(books_base, user_id=user_id, username=username)
     )
 
 
diff --git a/shelfmark/download/orchestrator.py b/shelfmark/download/orchestrator.py
index a653a010..48fc69b6 100644
--- a/shelfmark/download/orchestrator.py
+++ b/shelfmark/download/orchestrator.py
@@ -20,6 +20,7 @@ from shelfmark.core.logger import setup_logger
 from shelfmark.core.models import DownloadTask, QueueStatus, SearchMode, normalize_format
 from shelfmark.core.queue import book_queue
 from shelfmark.core.request_helpers import (
+    is_plain_email_address,
     normalize_optional_text,
     normalize_positive_int,
 )
@@ -90,11 +91,6 @@ _PROGRESS_BROADCAST_COMPLETE_PERCENT = 99
 _PROGRESS_BROADCAST_MIN_DELTA = 10
 
 
-def _is_plain_email_address(value: str) -> bool:
-    parsed = parseaddr(value or "")[1]
-    return bool(parsed) and "@" in parsed and parsed == value
-
-
 def _resolve_email_destination(
     user_id: int | None = None,
 ) -> tuple[str | None, str | None]:
@@ -106,7 +102,7 @@ def _resolve_email_destination(
     """
     configured_recipient = str(config.get("EMAIL_RECIPIENT", "", user_id=user_id) or "").strip()
     if configured_recipient:
-        if _is_plain_email_address(configured_recipient):
+        if is_plain_email_address(configured_recipient):
             return configured_recipient, None
         return None, "Configured email recipient is invalid"
 
diff --git a/tests/core/test_admin_users_api.py b/tests/core/test_admin_users_api.py
index 5a9107ab..ec2b8ce7 100644
--- a/tests/core/test_admin_users_api.py
+++ b/tests/core/test_admin_users_api.py
@@ -699,6 +699,19 @@ class TestAdminUserUpdateEndpoint:
         assert resp.json["settings"]["DESTINATION"] == "/books/alice"
         assert resp.json["settings"]["BOOKLORE_LIBRARY_ID"] == "2"
 
+    def test_update_settings_admin_is_not_limited_to_global_destination(
+        self, admin_client, user_db
+    ):
+        user = user_db.create_user(username="alice")
+
+        resp = admin_client.put(
+            f"/api/admin/users/{user['id']}",
+            json={"settings": {"DESTINATION": "/other/alice", "BOOKS_OUTPUT_MODE": "custom"}},
+        )
+        assert resp.status_code == 200
+        assert resp.json["settings"]["DESTINATION"] == "/other/alice"
+        assert resp.json["settings"]["BOOKS_OUTPUT_MODE"] == "custom"
+
     def test_update_response_includes_settings(self, admin_client, user_db):
         user = user_db.create_user(username="alice")
         user_db.set_user_settings(user["id"], {"DESTINATION": "/books/alice"})
diff --git a/tests/core/test_per_user_downloads.py b/tests/core/test_per_user_downloads.py
index 40f0e894..4e0ddfcb 100644
--- a/tests/core/test_per_user_downloads.py
+++ b/tests/core/test_per_user_downloads.py
@@ -5,8 +5,11 @@ Tests that DownloadTask has a user_id field and that the queue
 can be filtered by user.
 """
 
+import logging
 import sqlite3
 
+import pytest
+
 from shelfmark.core.models import DownloadTask, QueueStatus
 from shelfmark.core.queue import BookQueue
 
@@ -372,3 +375,143 @@ class TestTaskToDictUsername:
 
         result = _task_to_dict(task, current_status=QueueStatus.COMPLETE)
         assert result["status"] == QueueStatus.COMPLETE.value
+
+
+@pytest.fixture
+def destination_warnings():
+    """Collect warnings from the utils logger.
+
+    setup_logger builds its loggers outside the standard hierarchy, so caplog's root
+    handler never sees them.
+    """
+    from shelfmark.core import utils
+
+    messages: list[str] = []
+
+    class _Capture(logging.Handler):
+        def emit(self, record: logging.LogRecord) -> None:
+            if record.levelno >= logging.WARNING:
+                messages.append(record.getMessage())
+
+    handler = _Capture()
+    utils.logger.addHandler(handler)
+    try:
+        yield messages
+    finally:
+        utils.logger.removeHandler(handler)
+
+
+class TestUserDestinationContainment:
+    """A stored per-user destination only applies while it stays inside the global one."""
+
+    @staticmethod
+    def _config_get(global_values: dict, user_values: dict):
+        def fake_config_get(key, default=None, user_id=None):
+            if user_id is not None and key in user_values:
+                return user_values[key]
+            return global_values.get(key, default)
+
+        return fake_config_get
+
+    @pytest.mark.parametrize(
+        "override",
+        ["/config", "/books/../config", "/books/{User}/../../etc", "/booksmark", "../x"],
+    )
+    def test_override_outside_global_is_ignored_with_warning(
+        self, monkeypatch, destination_warnings, override
+    ):
+        from pathlib import Path
+
+        from shelfmark.core.config import config
+        from shelfmark.core.utils import get_destination
+
+        monkeypatch.setattr(
+            config,
+            "get",
+            self._config_get({"DESTINATION": "/books"}, {"DESTINATION": override}),
+        )
+
+        result = get_destination(is_audiobook=False, user_id=42, username="alice")
+
+        assert result == Path("/books")
+        assert any("DESTINATION" in message for message in destination_warnings)
+
+    @pytest.mark.parametrize(
+        ("global_destination", "override", "expected"),
+        [
+            ("/books", "/books/alice", "/books/alice"),
+            ("/books", "/books/{User}/fiction", "/books/alice/fiction"),
+            ("/books", "fiction", "/books/fiction"),
+            ("/books/{User}", "/books/alice/fiction", "/books/alice/fiction"),
+            ("/books/{User}", "/books/{User}", "/books/alice"),
+        ],
+    )
+    def test_override_inside_global_is_used(
+        self, monkeypatch, global_destination, override, expected
+    ):
+        from pathlib import Path
+
+        from shelfmark.core.config import config
+        from shelfmark.core.utils import get_destination
+
+        monkeypatch.setattr(
+            config,
+            "get",
+            self._config_get({"DESTINATION": global_destination}, {"DESTINATION": override}),
+        )
+
+        result = get_destination(is_audiobook=False, user_id=42, username="alice")
+        assert result == Path(expected)
+
+    def test_audiobook_override_is_checked_against_books_destination(self, monkeypatch):
+        from pathlib import Path
+
+        from shelfmark.core.config import config
+        from shelfmark.core.utils import get_destination
+
+        monkeypatch.setattr(
+            config,
+            "get",
+            self._config_get(
+                {"DESTINATION": "/books", "DESTINATION_AUDIOBOOK": ""},
+                {"DESTINATION_AUDIOBOOK": "/audio/alice"},
+            ),
+        )
+
+        result = get_destination(is_audiobook=True, user_id=42, username="alice")
+        assert result == Path("/books")
+
+    def test_audiobook_override_inside_global_audiobook_destination(self, monkeypatch):
+        from pathlib import Path
+
+        from shelfmark.core.config import config
+        from shelfmark.core.utils import get_destination
+
+        monkeypatch.setattr(
+            config,
+            "get",
+            self._config_get(
+                {"DESTINATION": "/books", "DESTINATION_AUDIOBOOK": "/audio"},
+                {"DESTINATION_AUDIOBOOK": "/audio/alice"},
+            ),
+        )
+
+        result = get_destination(is_audiobook=True, user_id=42, username="alice")
+        assert result == Path("/audio/alice")
+
+    def test_empty_override_uses_global(self, monkeypatch, destination_warnings):
+        from pathlib import Path
+
+        from shelfmark.core.config import config
+        from shelfmark.core.utils import get_destination
+
+        monkeypatch.setattr(
+            config,
+            "get",
+            self._config_get({"DESTINATION": "/books"}, {"DESTINATION": ""}),
+        )
+
+        result = get_destination(is_audiobook=False, user_id=42, username="alice")
+
+        assert result == Path("/books")
+        assert destination_warnings == []
diff --git a/tests/core/test_self_user_routes.py b/tests/core/test_self_user_routes.py
index 646f5fe2..d4fc34d7 100644
--- a/tests/core/test_self_user_routes.py
+++ b/tests/core/test_self_user_routes.py
@@ -301,6 +301,165 @@ def test_users_me_update_rejects_non_object_settings_payload(app, user_db):
     assert resp.json["error"] == "Settings must be an object"
 
 
+def _config_get_with_values(values: dict[str, object]):
+    def _get(key: str, default: object = None, user_id: int | None = None) -> object:
+        del user_id
+        return values.get(key, default)
+
+    return _get
+
+
+def _put_self_settings(
+    app, user: dict, settings: dict, config_values: dict[str, object] | None = None
+):
+    values = {"VISIBLE_SELF_SETTINGS_SECTIONS": ["delivery"]}
+    values.update(config_values or {})
+    client = _authed_client_for_user(app, user)
+    with patch("shelfmark.core.self_user_routes.load_active_auth_mode", return_value="builtin"):
+        with patch(
+            "shelfmark.core.self_user_routes.app_config.get",
+            side_effect=_config_get_with_values(values),
+        ):
+            return client.put("/api/users/me", json={"settings": settings})
+
+
+@pytest.mark.parametrize(
+    ("global_destination", "override"),
+    [
+        ("/books", "/config"),
+        ("/books", "/books/../config"),
+        ("/books", "/books/{User}/../../config"),
+        ("/books", "../config"),
+        ("/books", "/booksmark"),
+        ("/books/{User}", "/books/bob"),
+        ("/books/{User}", "/books/{User}/../bob"),
+    ],
+)
+def test_users_me_update_rejects_destination_outside_global(
+    app, user_db, global_destination, override
+):
+    user = user_db.create_user(username="alice")
+
+    resp = _put_self_settings(
+        app, user, {"DESTINATION": override}, {"DESTINATION": global_destination}
+    )
+
+    assert resp.status_code == 400
+    assert resp.json["error"] == "Invalid settings payload"
+    assert any("DESTINATION" in detail for detail in resp.json["details"])
+    assert "DESTINATION" in resp.json["message"]
+    assert user_db.get_user_settings(user["id"]) == {}
+
+
+@pytest.mark.parametrize(
+    ("global_destination", "override"),
+    [
+        ("/books", "/books/alice"),
+        ("/books", "/books"),
+        ("/books", "/books/{User}/fiction"),
+        ("/books", "fiction"),
+        ("/books/{User}", "/books/alice/fiction"),
+        ("/books/{User}", "/books/{User}/fiction"),
+    ],
+)
+def test_users_me_update_accepts_destination_inside_global(
+    app, user_db, global_destination, override
+):
+    user = user_db.create_user(username="alice")
+
+    resp = _put_self_settings(
+        app, user, {"DESTINATION": override}, {"DESTINATION": global_destination}
+    )
+
+    assert resp.status_code == 200
+    assert user_db.get_user_settings(user["id"])["DESTINATION"] == override
+
+
+def test_users_me_update_rejects_non_string_destination(app, user_db):
+    user = user_db.create_user(username="alice")
+
+    resp = _put_self_settings(
+        app, user, {"DESTINATION": ["/books/alice"]}, {"DESTINATION": "/books"}
+    )
+
+    assert resp.status_code == 400
+    assert user_db.get_user_settings(user["id"]) == {}
+
+
+def test_users_me_update_audiobook_destination_falls_back_to_books_base(app, user_db):
+    user = user_db.create_user(username="alice")
+    config_values = {"DESTINATION": "/books", "DESTINATION_AUDIOBOOK": ""}
+
+    rejected = _put_self_settings(
+        app, user, {"DESTINATION_AUDIOBOOK": "/audio/alice"}, config_values
+    )
+    assert rejected.status_code == 400
+    assert any("DESTINATION_AUDIOBOOK" in detail for detail in rejected.json["details"])
+    assert user_db.get_user_settings(user["id"]) == {}
+
+    accepted = _put_self_settings(
+        app, user, {"DESTINATION_AUDIOBOOK": "/books/alice/audio"}, config_values
+    )
+    assert accepted.status_code == 200
+    assert user_db.get_user_settings(user["id"])["DESTINATION_AUDIOBOOK"] == "/books/alice/audio"
+
+
+def test_users_me_update_audiobook_destination_uses_global_audiobook_base(app, user_db):
+    user = user_db.create_user(username="alice")
+    config_values = {"DESTINATION": "/books", "DESTINATION_AUDIOBOOK": "/audio"}
+
+    resp = _put_self_settings(app, user, {"DESTINATION_AUDIOBOOK": "/audio/alice"}, config_values)
+
+    assert resp.status_code == 200
+    assert user_db.get_user_settings(user["id"])["DESTINATION_AUDIOBOOK"] == "/audio/alice"
+
+
+@pytest.mark.parametrize(
+    ("settings", "expected_key"),
+    [
+        ({"BOOKS_OUTPUT_MODE": "ftp"}, "BOOKS_OUTPUT_MODE"),
+        ({"BOOKS_OUTPUT_MODE": ["email"]}, "BOOKS_OUTPUT_MODE"),
+        ({"EMAIL_RECIPIENT": "not-an-email"}, "EMAIL_RECIPIENT"),
+        ({"EMAIL_RECIPIENT": "Alice <alice@example.com>"}, "EMAIL_RECIPIENT"),
+        ({"EMAIL_RECIPIENT": ["alice@example.com"]}, "EMAIL_RECIPIENT"),
+        ({"BOOKLORE_LIBRARY_ID": -1}, "BOOKLORE_LIBRARY_ID"),
+        ({"BOOKLORE_LIBRARY_ID": "abc"}, "BOOKLORE_LIBRARY_ID"),
+        ({"BOOKLORE_LIBRARY_ID": True}, "BOOKLORE_LIBRARY_ID"),
+        ({"BOOKLORE_LIBRARY_ID": 2.5}, "BOOKLORE_LIBRARY_ID"),
+        ({"BOOKLORE_PATH_ID": "-3"}, "BOOKLORE_PATH_ID"),
+    ],
+)
+def test_users_me_update_rejects_invalid_delivery_values(app, user_db, settings, expected_key):
+    user = user_db.create_user(username="alice")
+
+    resp = _put_self_settings(app, user, settings)
+
+    assert resp.status_code == 400
+    assert resp.json["error"] == "Invalid settings payload"
+    assert any(expected_key in detail for detail in resp.json["details"])
+    assert user_db.get_user_settings(user["id"]) == {}
+
+
+@pytest.mark.parametrize(
+    ("settings", "expected"),
+    [
+        ({"BOOKS_OUTPUT_MODE": "email"}, {"BOOKS_OUTPUT_MODE": "email"}),
+        ({"EMAIL_RECIPIENT": " alice@example.com "}, {"EMAIL_RECIPIENT": "alice@example.com"}),
+        ({"EMAIL_RECIPIENT": ""}, {"EMAIL_RECIPIENT": ""}),
+        ({"BOOKLORE_LIBRARY_ID": "3"}, {"BOOKLORE_LIBRARY_ID": "3"}),
+        ({"BOOKLORE_LIBRARY_ID": 3}, {"BOOKLORE_LIBRARY_ID": 3}),
+        ({"BOOKLORE_PATH_ID": 0}, {"BOOKLORE_PATH_ID": 0}),
+    ],
+)
+def test_users_me_update_accepts_valid_delivery_values(app, user_db, settings, expected):
+    user = user_db.create_user(username="alice")
+
+    resp = _put_self_settings(app, user, settings)
+
+    assert resp.status_code == 200
+    assert user_db.get_user_settings(user["id"]) == expected
+
+
 def test_users_me_update_rejects_oidc_email_change(app, user_db):
     user = user_db.create_user(
         username="oidc-user",