diff --git a/src/kobo/handlers.rs b/src/kobo/handlers.rs
index eb72503..9a08f6f 100644
--- a/src/kobo/handlers.rs
+++ b/src/kobo/handlers.rs
@@ -347,12 +347,21 @@ impl KoboHandlers {
     ) -> Result<impl IntoResponse, (StatusCode, String)> {
         let user = resolve_user(&state, &auth_token).await?;
 
-        let book = state
+        let Some(book) = state
             .books
             .fetch_book_by_uuid(&book_uuid)
             .await
             .map_err(internal_error)?
-            .ok_or_else(|| (StatusCode::NOT_FOUND, "Book not found".to_string()))?;
+        else {
+            // The device is deleting a book that is not in our catalog (a
+            // sideloaded title, or a leftover from a prior library/server).
+            // DELETE is idempotent: the desired end state — "not on the
+            // server" — already holds, so acknowledge with 200. Returning 404
+            // makes the device treat the delete as failed and retry it on
+            // every sync, producing an endless storm of failing requests.
+            info!(book_uuid = %book_uuid, "Delete for unknown book acknowledged");
+            return Ok(StatusCode::OK);
+        };
 
         state
             .app_db