I'm not sure if you're having problems syncing or just getting your audiobooks appear in the audiobook section of itunes.
If you're ripping your own audiobooks from CD, they need to have the file extension of m4b and they will be added into that playlist. The m4b file is a file with bookmarking capability so you can listen to the book, leave the playlist, listen to something else, and come back to the audiobook and pick up where you left off.
Itunes doesn't directly do this but there are options out there for a free conversion to m4b - if you need help finding them, let me know.
Hope this helps.