LibriVox - free public domain audiobooks
OpenAudible - OpenAudible Project Page
Audible - Audible is the world’s largest provider of premium audiobooks and spoken word content.
DRmare Audio Converter - DRmare Audio Converter for Windows is a complete audio DRM removal tool designed to convert both protected audios, including Apple Music, Audible audiobooks, iTunes music, and common audios to MP3, WAC, FLAC, M4A, AAC, etc.
Mobilism.org - A home for book lovers, app enthusiasts, and much more.
TunesKit Audible AA/AAX - Audible AA/AAX Converter by TunesKit is an application that enables you to convert audible AA and AAX files to the formats that are supported by other devices.