iCloud deleted all my books

I had close to 100 books and documents in my iBooks. I could read books offline any time in the past, but since 2017 I had to download them time to time (not often). In 2018 I cannot access them without wifi at all, they always ask for a backup from the Icloud. I have never asked iCloud to freaking back them up. And when I turned off the so-called back up function, iCloud just deleted them from my phone. WHY? Is there a way to restore them? I used the general back up half a year ago, is there a way to restore them using it?