I just tried a few books that were not found or could not get the cover, even though they are both well known (over 50k reviews each on Goodreads).
The Physician https://www.goodreads.com/book/show/4692.The_Physician
Soumission https://www.goodreads.com/book/show/23929479-soumission
Is there something that might cause some popular books to not show?