If you like PlexAmp then you might be interested in Prologue [0] as well. It’s not a first-party app but it’s audiobook app that uses your Plex server as a source. So you can have a library of audiobooks that it pulls from.
I buy all my audiobooks on Audible, remove the DRM, then put them on my server so I can use Prologue instead of the default Audible app. Yes, you give up WhisperSync but I rarely use that anyway.