Aubile Audiobooks are distributed by Audible with the file extensions .aa and .aax. They contain data encoded with either the MP3 or ACELP codecs, but include a digital rights management (DRM) system requiring an Audible username and password for playback of the files.
A number of devices support playback of Aubile Audiobook files - for a full list, see the Audible Device Centre
Wikipedia reports that there is currently no support for Audible Audiobook files on Linux. However, Audible Manager does appear to work in Wine (per http://appdb.winehq.org/objectManager.php?sClass=application&iId=1612), although some issues with file transfer have been reported for the latest version.
The Wikipedia article mentions that a number of software tools (e.g. GoldWave are capable of re-encoding Audible Audiobook files. It also notes that .aa files can be burned to CD, removing any copy protection. Such CDs could then obviously be ripped and the files encoded in any appropriate format (OGG, MP3, FLAC, etc.).
There does not appear to be a published file format specification for Audible Audiobook files. pyaudibletags, which returns metadata contained in Audible Audiobook files, has some information on the file header.