The biggest problem with Apple Books (from a producer point of view) is that they removed the ability to connect dev tools to it. Up until the Catalyst rebuild you could open Safari dev tools, connect it to the Books instance, and use it to inspect CSS, highlight particular elements, and basically do anything you can in a browser.
This might not sound like much, but Books is unfortunately laden with weird bugs that don’t exist in normal Safari. They managed to fix the one I reported about srcset not working for images (it just didn’t display anything) but flex / grid display with 100vh is completely broken, and there’s a bunch more minor things like broken page-break functionality. And the debugging cycle currently is to make changes, repackage the book, reload the book in Books, and hope for the best.
This might not sound like much, but Books is unfortunately laden with weird bugs that don’t exist in normal Safari. They managed to fix the one I reported about srcset not working for images (it just didn’t display anything) but flex / grid display with 100vh is completely broken, and there’s a bunch more minor things like broken page-break functionality. And the debugging cycle currently is to make changes, repackage the book, reload the book in Books, and hope for the best.