The Case of the Great Train Robbery — Carl Brookins

A dog, spot, and his master, Alex Kava, find an ancient pistol in a hole beside their garage. The discovers sparks inordinate interest so Alex's father hires private investigator Sean NMI Sean to investigate. Sean is attacked in the Kava's yard and discovers many questions, about rotting money, the old pistol, and modern corruption. It takes all of Sean's considerable acumen to sort things out and bring a cop-killer to justice.