SOLUTION! -- I was getting this same screen after updating to iOS 13.4.1, found this thread, and tried everything mentioned here without success. I started thinking about what had changed and realized that I'd turned on "dark mode" during the upgrade process. I turned that off (in Settings->Display and Brightness), quit books and restarted, and it moved on to the next screen where it takes a few minutes to update your library. Once that was done, I was able to turn "dark mode" back on and could still access my library afterward. There's obviously a bug in the update process that causes it to crash when dark mode is on when it rebuilds the index for your library. Hope this helps everyone!