Early Lawrence Block from before he started using his sense of humor. A great hook here: Newly-wed virgins (I guess this was written at a time when this still happened) are about to spend the first night of their three week honeymoon at a remote cabin. Instead, they witness a gangland-style execution. The killers beat up the guy and rape the girl. They're young, scared and devastated. So what do they decide to do? Take the three weeks of vacation they still have and track down the killers and get their revenge. Whoo-hoo.
The evolution from naive kids to expert detectives and stone-cold killers is a little far fetched, but its a lot of brutal fun. Some of their investigation techniques, like calling the police and pretending to be an out of town reporter, are ones that Block uses again in later books as examples of bad sleuthing. I'm sure this was intentional on his part.
Lawrence Block has become my favorite genre writer. Even before he got down his style-chops and unleashed his sense of humor, he was cranking out this sort of delicious drugstore paperback work.