diff --git a/backend/src/chitai/services/dependencies.py b/backend/src/chitai/services/dependencies.py
index 310fb9e..63e5b6f 100644
--- a/backend/src/chitai/services/dependencies.py
+++ b/backend/src/chitai/services/dependencies.py
@@ -14,7 +14,7 @@ from advanced_alchemy.extensions.litestar.providers import (
 from advanced_alchemy.exceptions import NotFoundError
 from advanced_alchemy.filters import CollectionFilter, StatementFilter
 from advanced_alchemy.service import FilterTypeT
-from sqlalchemy.orm import selectinload, with_loader_criteria
+from sqlalchemy.orm import selectinload
 from sqlalchemy.ext.asyncio import AsyncSession
 from litestar import Request
 from litestar.params import Dependency, Parameter
@@ -51,8 +51,19 @@ from chitai.services.filters.book import (
 
 
 async def provide_book_service(
-    db_session: AsyncSession, current_user: m.User | None = None
+    db_session: AsyncSession, current_user: m.User = Dependency(skip_validation=True)
 ) -> AsyncGenerator[BookService, None]:
+    """
+    Provide a BookService with per-user data scoped to the caller.
+
+    `current_user` is a required dependency, not an optional argument. It used to
+    default to None with the scoping below wrapped in `if current_user:` — and
+    when it was not injected, that block was silently skipped, so
+    `Book.progress_records` and `Book.list_links` loaded *every* user's rows.
+    `Book.progress` returns `progress_records[0]`, so one user could see another
+    user's reading position; the shelf checkboxes leaked the same way. Failing
+    loudly on a missing user is the point of the change.
+    """
     load = [
         selectinload(m.Book.author_links).selectinload(m.BookAuthorLink.author),
         selectinload(m.Book.tag_links).selectinload(m.BookTagLink.tag),
@@ -60,31 +71,26 @@ async def provide_book_service(
         m.Book.files,
         m.Book.identifiers,
         m.Book.series,
+        # Reading progress, restricted to the caller.
+        #
+        # The restriction lives on the relationship via .and_() rather than in a
+        # separate with_loader_criteria(). advanced_alchemy's
+        # get_abstract_loader_options() keeps only _AbstractLoad,
+        # InstrumentedAttribute, RelationshipProperty and "*" entries and drops
+        # everything else — and with_loader_criteria() is none of those, so the
+        # previous criteria were discarded before reaching a query. A
+        # selectinload() carrying its own .and_() survives that filter.
+        selectinload(
+            m.Book.progress_records.and_(m.BookProgress.user_id == current_user.id)
+        ),
+        # Bookshelf membership, restricted to the caller
+        selectinload(
+            m.Book.list_links.and_(
+                m.BookListLink.book_list.has(m.BookList.user_id == current_user.id)
+            )
+        ).selectinload(m.BookListLink.book_list),
     ]
 
-    # Load in specific user-book data
-    if current_user:
-        # Load progress data
-        load.extend(
-            [
-                selectinload(m.Book.progress_records),
-                with_loader_criteria(
-                    m.BookProgress, m.BookProgress.user_id == current_user.id
-                ),
-            ]
-        )
-
-        # Load shelf data
-        load.extend(
-            [
-                selectinload(m.Book.list_links).selectinload(m.BookListLink.book_list),
-                with_loader_criteria(
-                    m.BookListLink,
-                    m.Book.lists.any(m.BookList.user_id == current_user.id),
-                ),
-            ]
-        )
-
     provider_func = create_service_provider(
         BookService,
         load=load,