I've found that iTunes no longer allows Audible files to be added to the iTunes library.
I'm on a Mac (OSx 10.9), and am running the latest version of iTunes (11.1.5).
First of all, there's no longer a "Books" category in iTunes, but there is an "Audiobooks" category. My preferences are set to show that category.
When I first tried to add my Audible books to iTunes, I had no audiobooks in my library. What showed when I was in the Audiobooks view was a sort of welcome, you have no audiobooks, page with a button to download audiobooks from the App Store.
I tried importing my Audible books via drap and drop and File-> Add to Library. I was able to select the files, but the Audiobooks welcome view didn't change.
I thought perhaps iTunes had a bug (or design) to only show the list of files when there was an App Store-purchased audiobook. So I bought an audiobook from iTunes. I was now able to see this audiobook, and proceeded to add my Audible file to the iTunes library as well. The Audible books again did not show up in my audiobook library.
Similarly, I've tried opening the .aa Audible file directly from Finder. It launches iTunes, but nothing happens.
There was another post about iTunes Match causing an issue, but I'm not subscribed to iTunes Match. https://discussions.apple.com/thread/3875987
So I'm out of ideas, and pretty fed up. Does anyone have a new way to fix this? Jimzgoldfinch method is not working. Books are .aa files