With the split of iTunes into four apps, the way audiobooks are managed is different. If you have audiobooks from Audible or from the iTunes Store – technically the Books Store – you have no choice: they can only be stored in the books app. But if you have a collection of audiobooks that you have ripped, or downloaded without DRM, then you have two options for managing audiobooks in a post-iTunes world.
You can move your audiobooks to the Books app, which offers a number of features for playback that are more appropriate for listening to spoken word. For example, you click buttons to skip ahead or back by 15 seconds, set a sleep timer, and more. However, these files are stored on your startup disk, and you may simply not have enough space on this disk, so if you have a large audiobooks library and want all your audiobooks in the Books app, I recommend only adding those to the app when you want to listen to them. At other times, store them on an external disk. (Audiobooks will be stored in a folder in the Library folder of your home folder: ~/Library/com.apple.BKAgentService.)
Or you can keep your audiobooks in your Music library. If you rip audiobook CDs, their files can stay in your Music library, and you can listen to them in the Music app, sync them to an iOS device, and even put them in your iCloud Music Library, if the bit rate is 96 kbps or above. This allows you to store the audiobook files on an external drive, if you don’t have enough space on your Mac’s startup drive.
Note that when you now go to rip new audiobooks, you must do this in the Music app; there is no such option in the Books app. But you can move these audiobook files to the Books app, and each file name shows up as an individual chapter, allowing you to navigate in your audiobooks more easily.
If you do want to keep them in the Music app, you no longer have to change the media kind to Audiobook for them to show up in the Audiobooks library, because that will be gone. You just leave them as music files, and they will show up in your Music library. It’s a good idea to set the genre to something like Spoken Word so you can find them easily.
So, if you do have a large audiobook library, make plans before upgrading to macOS Catalina.