Came back to zap this note because it sent me down the rabbit hole of downloading and self-hosting my Audible library. I used Libation to download, am hosting on my Plex server (Audnexus plugin for metadata), and Prologue (iOS) for my client.
This isn't exactly what you were looking for but, if you have an Audible account then you may be interested in downloading your audiobooks to your computer for safekeeping.
https://github.com/rmcrackan/Libation
I've used it and it worked great.
Discussion
Awesome. My next recommendation would be to ditch Plex for Jellyfin 🙂
I made that switch years ago and never looked back.