Click All Books in the sidebar, click Sort By (at the top right of your library), then choose Show iCloud Books.
If you see Hide iCloud Books, the books and audiobooks are already displayed.
Double-click a book or audiobook that has an iCloud status icon for a book or audiobook you want to add to your library.
You might be asked to enter your your computer needs to be authorized on audible.com.
Sign in to audible.com.
Find the audiobook you want to add to your library in Books, then click Download.
The audiobook is downloaded to your Downloads folder.
Open the Downloads folder, locate the audiobook, then do any of the following:
Click the audiobook.
Drag the audiobook to the Books app window.
In the Books app Read books in Books on MacAuthorize your Mac to get purchases from the Book Store, Audiobook Store, or the Audible website