I've been an Audible subscriber for a decade and have built up a sizable library over that time. I want to be able to back that library up in case it should become unavailable... for some reason.
Audible audio books are distributed with DRM, making them useless without the official app. To preserve the books, you need to break them out of this jail. A tool for doing that is Book Lib Connect.
The app let's you log in to Audible, and it'll list all the books in your library. Then you'll be able to download them all, and have them unlocked and converted to m4a/m4b files. Those files are playable in most competent media players on computers, and a bunch of apps on phones. The files are properly tagged with author, title, narrator and genre, and also includes cover art.
As I did this for my library of books, I noticed three books with the status License denied. Yeah. Turns out those books have been removed from Audible, now making them inaccessible to me. I'm already too late it seems.
I did also come across an issue where the downloads stopped with an error message saying something went wrong when inserting data into it's internal database. I restarted the download and it seemed to work fine. Hopefully that'll be fixed in the future.