StackSocial offers downloads of Ondesoft Audiobook Converter for Mac for free. That's $15 under our November mention and the best deal we've seen on this software. It purports to allow you to convert DRM-protected m4b, aax files into any of the most popular file formats on your Mac including MP3, AAC, AC3, AIFF, AU, FLAC, M4A, M4R, and MKA. It's compatible with OS X 10.6 and above.