The detective novel at its best is a morality play about knowledge. The detective knows something the rest of the world doesn't, and the plot is the process of revelation. This is a more interesting formal structure than it's given credit for, because it raises questions about what it means to know something and what knowledge obligates you to do.

Philip Marlowe in Raymond Chandler's novels is interesting precisely because his knowledge doesn't give him power. He knows what happened. He knows who did it. He usually can't do anything about it that matters because the people who did it have the kind of power that knowledge can't touch. This is a darker and more honest version of the detective form than the genre usually produces.

I reread The Long Goodbye every three or four years. It's the best American novel about friendship that's not ostensibly about friendship.