Audiobooks
Audiobooks on Android
Using Audioserve
Getting the App Download the apk file here: AudioServe-Android Sideloading APK: When you down...
Prologue for IOS
Get this app, and connect it to your plex account. I have a Library for Audiobooks there.
Booksonic
If you're still trying to use Booksonic, I'd really recommend switching over to Audioserve.