With the Books app taking over audiobooks in macOS Catalina, a lot of Mac users are upset that it doesn't handle large collections very well. You have no option to store your audiobooks on an external drive. However, you can still keep a large audiobook collection by managing the files yourself outside of Books, and only brining in the books you wish to listen to at the moment. Audible users may find it better to simply use the Auible iPhone app instead.