Welp, it looks like the Plex devs are already on it!
From that post:
Audiobooks? Ebooks? Other library types? This initiative will open the door to us adding support for these types of libraries and making sure that we have the appropriate metadata fields and library features
They’re thinking the same thing I am. Why is Plex limited to the types of data of “show” or “movie”? Why not something else? Audiobooks etc? Now we can start adding these ourselves!
We can finally solve the “TV shows and movies need to go together as part of the same canon” problem in a very abstracted way that allows the utmost in terms of customization!