You might want to check out Max Allan Collin's Quarry series. I've only read one book (Quarry's Ex) and I liked it a lot. I would like to read more.
There's also Lawrence Block's Keller series. I recently bought the first book, Hitman, but haven't read it yet.
I enjoyed Walter Mosley's Easy Rawlins and Fearless Jones series. I'm down to the final book and a book of short stories to wrap up the Rawlins series. I've enjoyed them all, and I have one book to go in the Fearless Jones series.
Chester Himes's Harlem Cycle books. I've only read one, The Real Cool Killers, and the two hero cops, Coffin Ed Johnson and Gravedigger Jones, were pretty hard core.