OpenAudible is “a cross-platform audiobook manager designed for Audible users.”
The software’s key feature is that it makes it possible to download audiobook purchases from Audible and then convert those to audiobooks to MP3 or M4B formats.
There is a big caveat to this capability–this only works with Audible books that users have paid for or purchased with credits.
About half my Audible library consists of free audiobooks, which Audible does not make available for download. OpenAudible returns a “No Library Record Found for user” for those books.
Currently, OpenAudible costs $19, which includes free updates for one year.