This year (February 14th, in fact) marks the 85th anniversary of the publication of The Maltese Falcon by Dashiell Hammett and has come to be known as the epitome of the hardboiled detective genre. In it we meet detective Sam Spade (who will forever be pictured as Humphrey Bogart in the 1941 film adaptation) and his partner Miles Archer are hired by a woman to follow Floyd Thursby. When both Archer and Thursby are killed, Spade is considered a suspect.
Hardboiled detectives are tough and gritty and have seen it all. They are no strangers to violence and have no intention of ever sugar coating the truth. Detectives like Sam Spade talk to the reader and let us know exactly what they are thinking at all times. Hardboiled is not only about a particular crime, it is also about how crime has affected society. Hammet wrote the Maltese Falcon in 1930 during prohibition, a time when stories of Al Capone and other gangsters filled the newspapers.
The Maltese Falcon is included in Modern Library's 100 Best Novels of the Twentieth Century.
Perhaps a little on the softer side is John D. MacDonald's Travis McGee. Like Spade, McGee is honest and self aware. He operates with a core of values, but has no hesitation to act with violence if necessary. He's tall, strong and physically fit, and describes himself as a salvage consultant, rather than as a detective. The one Travis McGee novel that I have read, and that was many years ago, was The Dreadful Lemon Sky. In this adventure, McGee receives a visit from a woman friend who is carrying a large amount of money. She promises him a portion if she does not return. Well, she doesn't return, in fact she is killed, and McGree and his friend Meyers find out why.
If dark, gritty stories with layered and intense protagonists are your cup of tea, you might also like to read novels by Dennis Lehane, Will Christopher Baer, and Christopher G. Moore.