I hear ya.
I have over 1100 audio books. When I imported them into "Books" and saw the mess left behind I was ready to scream. I had created a fairly elaborate playlist system using smart folders to keep track of what I had read, what I was reading and what was not read. I used the rating feature to do this.
All of that was gone. Now I was left with 1100 books in a lump in the barely functioning "Books" application. They had done literally nothing to make the application useful. 1100 books, most of them with that awful generic cover and essentially uneditable.
I had even gone through as I purchased them and entered in the short synopsis in the "comments" sections to help remind me of what they were. Now? The only way to do this is to use the "view list" feature and do it tediously one field at a time. Needless to say, you have to do the same to actually see the comment section as well.
It all reminds me of the very first data base programs of the late '80s.
The whole situation was sloppy to begin with. When Mojave rolled out all of a sudden I had two book apps. iBooks and Books. I couldn't delete one or the other obviously because they are a basic feature to the system. So when I called Apple about it, they told me not to worry about it and that it will be resolved a a later upgrade. So this was their solution.
How sloppy was that? And now, they've just gotten exponentially worse.
And to really make you irate, if you notice the way Books is set up on the iPhone it is set up largely to be a doorway to purchase books from Apple first and a book library for those you already own second.
I've been an Apple user now for over 15 years and I am growing tired of their attitude. I know there are a lot of Apple clones out their that are apologists for any slight mentioned against them - so bully for them. You would think Apple was their mommy and daddy or something.