Silly reason really. But if you have some audiobooks that are formatted as mp3, then you can’t just use iCloud Music Library. The reason is that files that are less than 96Kbps are marked ineligible and will not sync with iCloud, so you have to go back to the ancient (last century!) way of plugging into your Mac and it took me a while to remember how to do sync.
But the steps are plug in and authenticate, then click on the iPhone icon and choose Audiobooks and then Sync Audiobooks and Selected Audiobooks.
The super strange thing is that when you go to your I phone you do not go the the Music application, even though you added it with iTunes, anything marked
Audiobooks is actually automatically sent to the
Ibooks application if that isn’t super confusing.