Of course, if your interested in the "hard-boiled" school, you have to read Raymond Chandler and Dashiell Hammett, but the list hardly ends there. I've read one of Crais' books, which was OK; and Sue Grafton's Kinsey Milhone reminds me more of Jim Rockford than Sam Spade (not a criticism).
If you are interested in something odd, but squarely in the tradition, check out the Rex novels by Eric Garcia. Even though some of the characters are dinosaurs, the "hard-boiled" tone is perfectly done (much like Jim Lethem's Gun, with Occasional Music, though it strays a little towards parody).
Lawrence Block writes a series, or two, that are supposed to be "hard-boiled" but I can't recall the character's name. I think one of the titles is When the Sacred Gin Mill Closes.
Beyond that, I'll have to give it a little thought.