The Wrong Quarry (Quarry) by Max Allan Collins

He's not really a PI but a hitman. Still, in this novel of the series he does some investigating. Hired to put a hit on the hitman who intends to kill a dance instructor he also tries to find out who hired this hitman in the first place.
During this investigation he has sex with women of various ages and kills some people without any remorse. This is pure pulp, and I love it. It's pretty amazing how Mr. Collins can write about a cool and calculated killer who can think with his dick a bit too much and has no second thoughts about killing someone in cold blood and still make you like him in a way.
Pure, hardboiled fun. The influence of his Mr. Collins' idol Mickey Spillane is clear throughout this enjoyable crime story.