"Tales From the Dark Snow" by Brendan Dubois is influenced by Quarry. http://www.amazon.com/Tales-Dark-Snow-Brendan-DuBois-ebook/dp/B0055IH938
Read the Nate Heller novels by Max Allan Collins, too. They're about a PI, not a hitman, but they are similar in tone.
Anything by Max is great, actually. Even better, his books are cheap on Kindle.