I second the other suggestion for Prologue on iOS as a Plex frontend for audio books. It’s such a good experience that I’m now listening to more audio books than before and they also just launched an Apple Watch version.
It’s sad how true that is for many things these days. I usually download shows to have them in Plex even though I have many paid streaming accounts. Just because it’s a better UX and it also automatically scrobbles to Trakt.tv.
Interesting, I'm going to have to check this out. I've been using OpenAudible for this purpose (paid).