Smart Converter and All2MP3 don't handle the .aa audiobook format, unfortunately.
I just searched for a couple of hours for a converter which handles audiobooks, and found nothing free. However, there are other ways.
Here are the three ways which I've found to do convert .aa to mp3:
1.
If your audiobooks are from Audible, use your friend's Windows computer: convert them using Audible's listening software for Windows. This seems to be the easiest way.
2.
If you have time to let the audio convert in the background, you can let it play into an internal audio recording software such as Soundflower (which is free, and found at
http://cycling74.com/products/soundflower/). To use this, open your audio book in iTunes. Follow Soundflower's directions to record the audio - saving it as an mp3 file - as it plays. Note that it does not have to play through the speakers: Soundflower counts as an audio output device. You might not be able to use iTunes to play anything else, while it's converting. As far as I know, this records at the speed that the audio plays (so that a 14-hour audiobook would take 14 hours to be converted to an mp3 file). So, this is best done overnight or while you're out.
3.
Burn your audiobooks to a virtual (or real) CD, then import them from the CD. Thanks to blevins321 for this. (
https://forums.macrumors.com/threads/1525280/). This is a bit time-intensive, but it works.
A good virtual CD software is
http://www.burningthumb.com/apps/virtual-cdrw/. This is only free for the first 15 days.
In order for iTunes to recognize the CD, you need to burn it using iTunes (not Finder). Follow the instructions in this video:
https://www.youtube.com/watch?v=7vO_uVsxxvo. For a typical audio book, you'll need many virtual CDs, since iTunes assumes they're of a normal CD audio length (about 80 minutes). Make sure you set the time between tracks to 0s, since iTunes breaks the audiobook into 8 minute tracks.
As you re-importing the files as mp3 files (following the video directions), watch out for iTunes' naming conventions and organize the multiple CDs' files how you wish. I found it simplest to organize them after each CD import, so that I didn't get confused about track order. (I put each imported CD into its own folder, so that I didn't have to rename every single track.)
For a 14-hour audiobook, there are 12 CDs and just over 100 8-minute tracks.
Good luck!
If you're finding this all a bit messy, I highly recommend writing to whoever your audiobook provider is (such as Audible), asking them to supply a free mp3 converter for mac users or a downloadable mp3 version of the books.