For a while now I've been importing mp3s, setting them as "Audiobook" and listening to them via iBooks, like normal. However, for some unknown reason a few weeks ago, this stopped happening: Even though they show up in "Audiobooks" on iTunes, they WILL NOT show up in iBooks anymore—they still appear in the Music app, like a regular mp3. Nothing I have tried will force it to show up in iBooks again.
Edit, to clarify: The problem is that I add an MP3 audiobook to my iPhone via iTunes in the normal matter, but it shows up in "Music" and NOT "iBooks" EVEN THOUGH I set the file type as "Audiobook." iTunes—on multiple computers!—correctly shows the audiobook, but not on the phone. Furthermore, I deleted an old audiobook on the phone yet iBooks STILL shows it!
This is on iOS 10.3.3, iTunes 188.8.131.52, and reconfirmed on iOS 11.2.2, iTunes 184.108.40.206.