As you have noticed already, Plex has no explicit support for audio books.
Which means you are using the existing support for music.
But that means you will have to bend Plex a bit to do what you want.
And that will only work reliably, if you make use of embedded meta tags in your audio files and put them into a supporting folder structure.
And then you need to instruct Plex to prefer the information from your meta tags, instead of primarily trying to find suitable information on Musicbrainz.
See Audiobook wrong metadata - #2 by OttoKerner
This works best if you create a wholly separate section for audio books, instead of trying to combine music and audio books together.
