You're not meant to judge a book by its cover, but that rule goes out the window with crime novels. Frequently the cover is the best part. It started with the pulp novels, which would put a lurid image with lots of cleavage and knives and looming death on the front in order to sell a cheap story. It worked. So much that the style has now been appropriated by 'proper' books. But not all crime novels have pulp covers. Some are modern and minimal, some solely text-based, but any good example needs a sense of intrigue and excitement.
Here's a gallery of some of the most spectacular covers in crime fiction, from the beautifully tacky to the just plain beautiful.