Both deleted and new audiobooks show in iTunes, but not on iPhone
Found some old (iphone 8) information that suggested it's a syncing problem related to manual syncing, and led me to figure out this answer for the iphone X.


Disconnect iPhone from iTunes.
In iTunes, delete the unwanted book from the library and move it to the recycle bin. (Note: This step might not be necessary, but I did it as an extra precaution/preemptive step. If I wanted to keep the book, then I'd just have to remember to retrieve it from the recycle bin.) 
Still in iTunes, go to Edit > Preferences > Devices
Uncheck "Prevent...from syncing automatically"
Reset Sync History
Click OK.
Reconnect iPhone to iTunes. Wait for the synching process to complete.
In iTunes, edit preferences to manually sync again (assuming this is the state you want to revert to).


Note: I was nervous about auto syncing because I thought it might transfer all music and all audiobooks to the phone, which I definitely did not want. Am happy to report that in this case, the only audiobook synched to the phone was the one I'd already transferred.