U is for Undertow (2009) written by Sue Grafton

In April 1988, Kinsey Millhone is hired by a young man named Michael Sutton to investigate a memory that he claims to have recovered of two people burying a body in the woods in the exclusive Horton Ravine neighborhood in 1967, when he turned six years old. He claims that the burial took place on his birthday, two days after a famous unsolved kidnapping of a four-year-old named Mary Claire in the neighborhood, in which the kidnappers requested $25,000 but never picked up the ransom after police were called. She was never found. Although Michael's memory is hazy, he and Kinsey manage to locate the spot of the burial, but a police dig uncovers only the body of a wolfdog named Ulf. One of the bystanders watching the dig, a banker named Walker McNally, who was a high school classmate of Kinsey, spends the entire weekend drinking and kills a young woman in a DUI on Sunday night.