Someone sent in a comment asking me to recommend some more obscure paperback originals. I have two titles that I always mention. One of Fletcher Flora's The Hot-Shot, which you can pick up on abebooks for a fairly reasonable price. It's not a mystery novel, but it does have some criminous elements. The main thing it has going for it is that Flora was channeling Holden Caulfield when he wrote it. The Hot-Shot is the best pastiche of The Catcher in the Rye that I've ever read. My theory is that Flora was just so knocked out by the voice in that book that he couldn't resist giving it a try. You'll have to judge for yourself how successful he was, but I liked this one quite a bit.
The other title that I like to remind people of is Revenge by Jack Ehrlich. It's also available on abebooks. We're talking Jim Thompson territory here, and if you like The Killer Inside Me, this one should definitely be on your list. A more recent Ehrlich title, Bloody Vengeance, is well worth looking for. It's about a different kind of revenge, but it's a powerful statment. I'm not sure how serious Ehrlich was when he wrote it. You'll have to read it to see what I mean. It's really cheap on abebooks.
As always, remember that my recommendations should be taken with a grain of salt. I have no critical sense. I just know what I like.