What happened to my books in iPad update?
I had a similar problem since iOS 13, but only on my iPad not my iPhone.

I have about 500 books and I like the bookshelf view. After the iOS 13 update, several of the books disappeared. The rectangle for the cover view was still there, but it was blank. When I clicked on it, nothing happened except that the app froze.

The problem seemed to be with books on iCloud, not those stored locally.

I tried various things:


Switching off syncing for Reading now brought them back - for a short time
I deleted the app and reinstalled it. The books displayed correctly as the books downloaded from iCloud but when downloading was complete, they disappeared again.


In the end, changing to the list view seems to be a workaround! The icons are still blank, but the details are shown in the list and the books will download if I click on them.