The website neatopotato has a bunch of cool adult pulp novels for download, I recommend you check them out. They're easily convertible to your Kindle or Kobo and free.
Three of the books have no covers, and the cover is half the fun of these books. I looked all over the net and couldn't find them, so here's two of the missing covers, complete with links.
Blow the Man Down by Jason Bonds
Glory Hole Cop by Barton Lewis