Apple, in an email to book publishers:
Thank you for being a member of the iBooks Author community. We have some news to share with you about the future of book creation.
Two years ago we brought book creation into Pages. With key features such as the ability to work on iPad, collaborate with others on a shared book, draw with Apple Pencil, and more, Pages is a great platform for making books.
As we focus our efforts on Pages, iBooks Author will no longer be updated and will soon be removed from the Mac App Store. You can continue to use iBooks Author on macOS 10.15 and earlier, and books previously published to Apple Books will remain available. If you have iBooks Author books you’d like to import into Pages for further editing, we have a book import feature coming to Pages soon.
There is a corresponding support document with more information.
While Pages doesn’t have every feature iBooks Author did, iBooks Author was buggy and slow. If I were to take on another book project, I don’t think I’d miss the old application.