"Old book/film covers/posters are great because they often don't "do" subtlety :)"

Exactly - for a more "pulpy" looking image, the colors are too dark and muddy - they need to be more varied and contrasty to each other to stand out on their own - but not "rich" per-se - like a medium blue against a medium orange - get it? As it is, the image doesn't have nearly as much separation between the foreground and background elements so that they can stand on their own.

There's a "playful garishness" to pulp covers. Cross-lighting with strong highlights and defined shadows. Using COLOR for separation as well as lighting, etc.

As far as the surrounding borders/edges, scan or photograph creases in papers and put them all around the edges to make them look worn and read again and again and again - and since it's cheap "pulp" paper it suffers from bad folding after repeated use.

You want a great example of how to do it well, grab a copy of the movie Pulp Fiction and look at the poster art - I know this is bluntly obvious, but they did a fantastic job!

Hope this helps - it's a great start, but needs more push. Good luck and have fun!
-Lew ;-)