Incomplete downloads and disappearing content in Books on Apple devices

I keep most of my things in “iBooks” since I first got Apple Products way back when. I have called about this issue before, and the Apple Rep helping me was dumbfounded, as to why it wasn’t finishing. That was like 3 iPhones ago. Needless to say, things are still not being saved completely, and sometimes when I want to open various ones, when I click on them, they completely disappear. 🫥 some of these are older, and no way to find out where to get them again. This issue has been here 3 years or more, truth be known, about 6-7 years. Anyone else having these issues?? I need some help… please.

[Re-Titled by Moderator]