A few days ago I was making a video with my iPad Air 2 camera. A pop-up alerted me that the 32GB memory was almost full, inviting me with a pop-up "Manage your space" or like this in Italian "Gestisci Spazio". I clicked the pop-up and was asked me if I would delete apps that I don't use so often. I accepted and Ipad deleted some apps. OK I said.
The day after I realized that the Books app was without my 80 books previously present! Ipad had deleted all my books whithout asking me! Very frustrating :-( . All my books (most of them pdf) had been downloaded from safari or emails so not from the apple store. A learned that the backup doesn't include pdf from Books, so a cannot restore an old backup I done in October 2020. I didn't have activated the iCloud Books switch in iCloud menu under settings, so not recovery is possible.
I already asked apple support without help at all. I read that I'm not alone with this big big bug.