There is no official support for audio books in Plex.
As such, you need to reappropriate some of the metadata fields for audiobook use, while still adhering to the rules for music in Plex.
In general, I urge you to create a separate library of the type Music for all your audiobook/podcast content.
Which then allows you to enable the checkbox “store playback progress” in the properties of that library. (it should NOT be enabled for a regular music library)
This setting should store the playback progress of the currently playing file and resume it at the right place if you play it again.
It will also enable automatically special podcast features in Plexamp (Plex Pass required).
You will need to rely on embedded meta tags in your audio files.
Keep in mind that the folder structure and the AlbumArtist are very important in Plex.
See Audiobook wrong metadata - #2 by OttoKerner for more details.
