Omega Dog, by Tim Stevens

Our hero is private investigator Venn.  A successful military officer turned police detective, his violent impulse towards presumably guilty suspects causes him to be fired, and he ends up in New York as a sad-sack private eye.

Things get interesting when he's framed for murder and a mysterious government official offers him a way out - if he finds a missing scientist.  You're probably asking yourself, "but where's the beautiful woman for our hero to hook up with?"  Oh, don't worry - there's a damsel in distress as well.

It was actually a pretty good story.

Omega Dog (Joe Venn #1)