iBooks Author will no longer be updated, and it won't be available to new users as of July 1, 2020. If you’ve previously downloaded the app, you can still access it from your App Store purchase history. You can continue to use iBooks Author on macOS 10.15 and earlier, and books previously published to Apple Books from iBooks Author will remain available. If you have iBooks Author books you’d like to import into Pages, a book import feature is coming to Pages soon. It will allow you to open and edit iBooks Author files (.iba) in Pages.
Click here to learn more about book creation in Pages.
iBooks Author can still be downloaded from the App Store at the link below.