More than once it has been asked how to make Audiobooks that they imported on CD to show up in the Audiobooks category in iTunes. Here are the step by step instructions:

1) I'm assuming you have your audio files in iTunes already, if not go
ahead and import them at the desired quality. I suggest that when you
do this you have your iTunes preferences set to keep your iTunes
folder organized and copy files to your iTunes Music folder when
adding to their library.

2) Now you need to change the extensions on all of those audio files.
You'll want to find the location of where all the files are because
you'll also use that in step 4 below. If you keep your iTunes library
organized, they should be in your iTunes music folder in the
appropriate sub-folders. You can also select a track and right-click
and choose "Reveal in Finder." Remember this location, or keep the
window open in the background, you'll need it in a minute.

 The easiest and most user-friendly way to convert the file formats is
to use the "Make Bookmarkable" script from Doug's iTunes Scripts
(http://www.dougscripts.com/itunes/) simply download the script and
follow the directions to install and run.

3) After the script has run, it will appear in iTunes that nothing has
happened. That's because the script has made changes to the actual
files and iTunes needs to be updated. To do this highlight all of your
audiobook files and remove them from your iTunes library. However- DO
NOT DELETE them from your computer. The easiest way to do this is to
select all the audiobook files and press the delete key on your
keyboard. When it askes you if you would like to move these items to
the trash- select NO (if you accidently say yes, that's fine just
don't empty the trash!)

4) Now we have to pull those files back into iTunes. Remember where
those files are located (back in step 4) and take those original audio
files and pull them back into iTunes. Simply select them, and drag
them into an iTunes window.

That should be it- Good Luck!