Prologue is the best (and possibly only) audiobook app for Plex.
FEATURES
– All the benefits of Plex: stream your audiobooks from anywhere in the world
– Pick up from where you left off: prologue remembers playback positions across all your audiobooks
– Download your audiobooks to listen when you’re offline*
– Organize your audiobooks by grouping them into collections*
– Bookmark audiobooks that you plan to listen to later
– Sleep timer and playback speed control
– CarPlay support
* Feature requires a one time purchase to unlock