iTunes does not allow me to select some audiobooks for sync

Workaround:

Copy your audiobook files to a location outside of your iTunes library folder.

Delete the issue audiobooks from iTunes, and select to move files to trash.

Import the copied files back into iTunes

Alternate workaround:

Convert audio files to a different format following instructions here.

New songs appear in library as a music album. Find the album in your library.

Delete the original Audiobook album from your library

Select the new music album of your book, press Command + I

Go to Options tab > set Media Kind Audiobook

I spoke to Apple support. They are aware of this problem that was caused when they changed something about the way iTunes manages files a few years ago. There is no universal solution. You must individually re-import or convert every file to get them working again.

Just noticed a pattern that all my audiobooks older than about 1.5 years are the ones that I cannot sync.