There's a RainbowCrack plugin which allows you to extract your activation_bytes which is both fast and free. Once you're extracted your activation_bytes, it can be saved and re-used to remove the DRM from all your audiobooks.

Then you can use ffmpeg to convert the .aax file:

    ffmpeg -activation_bytes $activation_bytes -i $audiobook_file -c:a copy -c:v copy $output_file

I'd suggest converting to m4b, which you can achieve by replacing the aax extension with m4b as part of the output_file. This preserves all chapters, metadata, and even the cover art. The m4b format also has a neat feature which lets you bookmark your last position in the same file.

Ah well, I bought the program years ago, and it is pretty convenient—just drag and drop. So I expect I'll continue using it. It has the option to use m4b files too, so maybe I'll try that.

I searched for days before I bought it though, and never came across the ffmpeg method. All sorts of hacks involving virtual CDs and whatnot, but nothing that scaled.

Keep your $30. Or... Pay me $30 :)

ffmpeg -activation_bytes 1CEB00DA -i test.aax -vn -c:a copy output.mp4

Does this leave enough meta data for other book readers? You can listen to them in a generic audio player but I like having something that understand the chapter breaks.

You have to copy the video as well if you want to keep all the metadata, chapters, and cover art. I'd suggest converting to m4b rather than mp4. I gave an example in my other comment in this thread.

Does this really work?

Yes, it does. You'll have to use the correct activation bytes to decrypt, but those can be retrieved by signing into the audible account.

How do I read those out when logged in?

Probably easier to just plug my phone into the aux input and record with audacity.

Not really. The hardest part is getting the activation bytes and even that is actually quite simple and you only have to do it once. For the actual converting I have a bash script that can convert all the aax files in a directory and convert them to a m4b format with all the metada intact. Using your approach you would have to manually set the chapters and the cover art. I have thousands of hours worth of audiobooks but even if you only have a single 10 hour book I think this approach is still the easier of the 2.

Since breaking DRM is itself illegal, why not just torrent the content?

Because they're not both illegal under all jurisdictions. IANAL but breaking DRM for personal use is legal in the US.

Check out OpenAudible.