According to Max Allan Collins' afterward, This book was the first to be written of the Quarry series. It was re-released a few years later by his publisher with a different title: The Broker, a change he was unaware of, so when the opportunity to bring it out once again came around, he changed the title back to its original.
I like the Quarry series, but you can tell this was an early, unpolished work. Quarry's character is unsettled, and you get the feeling that Collins was struggling a little to make him into a bad guy with few redeeming traits. Collins admits he was "ripping off" Westlake's Parker, but in this book (not so much in the later Quarrys) the protagonist lacks Parker's sense of irony, as brutal as he may be.(See the author's correction below.)
Quarry is sent by the Broker to kill an apparently innocuous man in a small town along the Mississippi (patterned after Muscatine, Iowa but we don't get a good sense of place). He's also charged with taking out a man at the airport who is carrying a load of heroin. Quarry abhors having anything to do with drugs so he sets up the Broker who ordered the hit by hiding half of the load which he later uses to his advantage. When things start to go wrong, contrary to all good sense, he decides to find out who the original purchaser of the hit was.
It's a fun read, but not up to the level of the later books in the series. For a truly memorable "hitman" series read Lawrence Block's Keller books which set the standard.