The Bone Polisher (Simeon Grist) by Timothy Hallinan

This Simeon Grist novel, now available as an ebook, came out in paper a decade ago, which makes some elements (the technology, the role of AIDS) a bit dated. Still, that doesn't hamper the perfect hardboiled mystery that incorporates these.
Simeon is hired to find out who killed an older, kind gay man and discovers there's a serial killer at large who's killing gay men.
Simeon is just a fantastic PI. Every bit as cynical as Marlowe, but probably more unsatisfied with who he is. We follow his struggle with what he sees as his meaning of life, the way he feels he's only effective as a PI these days. Another thing that makes him so cool is that he lacks the cop or military background of most PI's. As he tells another character he's self-taught.
The amount of action is just right, the personal drama makes the story rise above most PI novels and the weird climax in a Halloween setting is unique.
Fantastic novel!