A Ross MacDonald is like an extremely well crafted 1950's black & white noir movie. Nothing comes through in it's true color, everything is projected in shades of gray, the action is stately yet never drags, and the characters are all vaguely threatening.
All of MacDonald's novels exhibit certain basic themes--tormented families, buried secrets that fester through multiple generations, environmental destruction, and the brutal contrast between rich and poor. The key to MacDonald's long running success was Archers realism and authenticity, MacDonald's ability to craft complex yet understandable stories, his mastery of language, and his ability to generate a specific atmosphere of threatening suspense on a consistent basis.
All of the above referenced themes are present in The Drowning Pool, which I think is MacDonald's best novel, though The Underground Man is right up there as well.
MacDonald's novels aren't just mind candy-reading him is a literary experience. I believe that is why he was successful in a sort of restrained way. Escapists will not get into these books-they are too cerebral. If you want to your books affect you, MacDonald and Archer are your kind of guy's.