Some good ones here!
Georges Simenon (1903-1989) was one of the most prolific writers (and shaggers) in history, with almost 200 novels and 150 novellas (and perhaps 8,000 prostitutes) to his CV. Even more startling, most of these novels and novellas are actually very good, with a number of them being small masterpieces.
Simenon is probably best known for the Inspector Maigret novels, a series of low-key detective stories about a wise but not super-brilliant Parisian cop. One of the US covers to this series featured in my first proper post. When Penguin UK reissued 14 of the novels a few years ago, they went with a look that is clever and eye-catching. Here are 9 of those covers (click to see enlarged).
Each graphic element on each cover refers to something from the book--a clue, a red herring, and aspect of Maigret himself, or a location. They're brilliant. Unfortunately, they've also been recently replaced a much simpler, more boring covers.
Penguin also publish several of Simenon's non-Maigret novels as well. These covers have a nice Brassaï/film-noir feel.
Finally, there are the non-Maigret books being put out by NYRB Classics. A number of these also have a similar atmosphere (and you can see the rest here).
UPDATE: The set of 14 Maigret covers discussed above were designed by Jamie Keenan.
UPDATE 2: The redesigns I describe as "more boring" are actyually by the usually wonderful David Pearson.