The story proves the thematic/moral argument.
As this story expands, the theme might shift, because there's a lot of room for it, but if it remains, then it would be interesting to see the many different ways that people misjudge a wide range of books by their covers.
I'm reminded of Blood Simple, the brilliant neo-noir film by the Coen Brothers.
Anybody know the theme to THAT film?
"Don't make assumptions."
That's it. Period.
Because from the first frame to the last, every character who makes an assumption is wrong wrong wrong.
If you don't recall it, watch it again with this in mind, and you'll be amazed at how well the Coens crafted the story to blend with an otherwise typical film noir-styled story.