I have a lot of books in my library on my desktop. Since probably iOS13 I have been unable to see any books at all on my elderly large iPad2 and most books on my smaller iPad. But nothing new can be persuaded to move to the one of these that allows me to read books. Result, that I am now unwilling to purchase (or acquire by any means, it turns out) books, since one iPad is now useless for that and the other will not accept any changes in the library. I can see very many books on the desktop but nothing transfers by any means suggested on help pages.
I do not want stuff in the cloud for several reasons, the biggest of which is that the ability to read books is most useful when there is no available wi-fi connection. So all routes to a suction via the cloud are irrelevant. There is no problem transferring photos or mail or any other material that I'd want shared across platforms - this is quite clearly a book-related problem.
On the larger iPad I get to see a screenful of books in the library, for about the 30 seconds it takes for the books app to crash and return me to the home screen.
Having been an Apple user since the early 80s, I found this desperately disappointing. The iPad as a reading medium was so good I got rid of a lot of paper. This position makes me feel that was a mistake.
Self-evidently, no posted help yet seen fixes my problems.