I've used https://openaudible.org/ to download my Audible library before, if you want a fairly simple GUI.

If you don't want to pay $12 for a GUI nor use Docker:

1. This tool can be used to extract your encryption keys: https://github.com/inAudible-NG/audible-activator

2. ffmpeg -activation_bytes XXXXXXXX -i name.AAX name.mp3

If you're doing that you might prefer to output to opus instead. Speech compresses well so you get the benefit of smaller files and have more space for other media. Just replace name.mp3 with ..." -c:a libopus -b:a 24k name.ogg"

If you're going to use a less compatible format than mp3, it is far batter to just swap the container and not transcode it:

-i in.aax -c copy out.m4b

1. does not transcode 2. captures and retains all of the metadata

Yes, I'm usually strongly opposed to lossy transcodes, but for portable listening of speech heavy content on a space-limited device, that plays Opus easily, it can be a worthwhile trade-off for many.