Play dead - David Rosenfelt

Andy Carpenter rescues a golden retriever from being put down and finds that the dog was supposed to have died five years before during an incident that resulted in his master being convicted of murdering his fiancee at sea.

Encouraged by the prisoner's sister, Andy endeavours to reopen the case by using the dog as a witness in court.
He finds himself the object of violent interest by persons unknown who very much do not want a new trial, resulting in car chases, attempted assassination and dog-napping.
Andy displays his usual court-room theatrics, but he has to bring in the Mafia before he gets to the bottom of a remarkable chain of impersonations and murder.