Just played around with audiobooks on my #oneplus6 with #postmarketos. There doesn't seem to be a nice Audible client atm., even though it might be possible to make one. Then I learned about https://librivox.org/ which has several thousands Public Domain audiobooks - awesome!
So first I tried Cozy - pretty nice app but it requires downloading things first. Then I figured that the page also has RSS feeds - just like most podcasts. So I tried the Podcasts app:
Using Podcasts with audiobooks as podcast "show" seems to work pretty well and I wonder now if the search/discovery feature and RSS support could somehow be ported and adapted to Cozy.
P.S.: regarding discovery / browsing available audiobooks: maybe Shortwave is the better example here.