What is the recommended way to make a backup of all the books I have purchased with Apple books (formerly known as iBooks), preferably using macOS?
Or may there is even an official way recommended by Apple?
Apple’s recommended method for any local backups is Time Machine. While other options exist, none of them match your combined wishes in question.
When you select the options for the book (
...), you can
Share PDF and simply e-mail it, save to files, etc.