Books Art Not Downloading from iCloud
So, I was facing the same problem and it was super bugging me. I have "kind of a fix": instead of uploading the book through Mac OS, do it on the iPhone.


Delete the book from all devices and iCloud.
Put the .epub or .pdf file in a folder on iCloud (just like any other file, like on the Documents folder)
Click on it on your iPhone.


It will open Books and sync it with the cover! But I had the problem that, if I had Books open at the same time on my Mac, the book would disappear. So I just left Books open on iPhone, did all the books, and then opened on my Mac. It worked fine!!

Where I got it: https://www.reddit.com/r/MacOS/comments/dov3x4/books_wont_upload_book_covers_to_icloud/