Pro tip: If you have #audiobooks purchased through Audible, you can download and back them up for play outside of Amazon ecosystem using OpenAudible: https://openaudible.org

The app costs a small license fee but I highly recommend it. It downloads the books in proprietary aax format and then you can use the same app to convert to m4b, m4a, mp3. The m4b/a formats have support for chapters that work in iBooks and Emby (that's the media server I use). I imagine Plex has some kind of support for audiobooks, too. Of course mp3s you can play nearly anywhere.