The funny thing about iTunes is, unless you buy Audiobooks from iTunes store, they insist on showing up inside of your main music library.
Making them show up inside the "Audiobooks" tab, so you can choose to synch book by book to your iPhone, is actually fairly simple.
Here are the steps -
- Change import format of iTunes to AAC.
- Select the MP3 versions of the audiobook, right click, convert to AAC.
- Go to windows explorer, rename the .m4a's to .m4b's. Copy them somewhere else on your disk.
- Delete them from iTunes library.
- Re-add them to your iTunes library.
Now they should show up inside the Audiobooks tab.
You can use the same technique for custom/free ringtones on an iPhone.