Audible Audiobook

Description
.aa files are used by Audible to distribute audiobooks. 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.

Hardware
A number of devices support playback of .aa files - for a full list, see the Audible Device Centre

Software

 * Audible Manager
 * iTunes
 * Windows Media Player

Wikipedia reports that there is currently no support for .aa 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.

Conversion
The Wikipedia article mentions that a number of software tools (e.g. GoldWave are capable of re-encoding .aa 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.).

Information
There does not appear to be a published file format specification for .aa files. pyaudibletags, which returns metadata contained in .aa files, has some information on the file header.