The Drowner — John D. MacDonald

Events are set in motion by the death of Lucille Hansen who drowns in a lake. The cause of death is a mystery. Some claim it was accidental, although Lucille was a strong swimmer, while still others believe it was a suicide as she was on the verge of matrimonial breakup and was having an affair with Sam a local speculator. Her sister, Barbara believes it was murder and engages the services of a detective agency to uncover the truth. Assigned to the case is Paul Stanier who -- posing as an insurance investigator -- digs into the case. He finds a society of shallow and voracious people and not a little corruption with Sam in particular having used Lucille to hide a cache of money he was anxious to conceal from the IRS and his own somewhat shady tax lawyer. The money is missing -- which points to murder.