Since this is no music at all, chances are slim that the Premium Music agent recognises it successfully.
Plex has almost zero support for audio books. All you can do is some workarounds and a special library configuration. So I recommend you to actually create an additional, separate audio library of the ‘regular’ (last.fm) type.
This ensures that you don’t get an audiobook chapter when you just want some random music.
And it allows you to use a totally different metadata agent for your audio books.
For the rest, use this as a starting point: Audiobook Guide
