New Tricks by David Rosenfelt

Andy Carpenter is designated the caretaker of a Bernese dog who is the middle of a custody battle after his owner is murdered. Waggy's owner, Walter Timmerman, was a chemist who made his millions in the pharmaceutical industry. Right after he's appointed, Walter's wife, Diana is killed when a bomb blows up her home. Walter's son Steven disliked Diana (his stepmother) and is soon arrested for her murder. Andy decides to take him on as a client. But when Andy's girlfriend, Laurie, is shot, he realizes that someone wants Waggy dead. But what kind of threat does the dog really pose...and to whom? New Tricks is another humorous, light mystery in the long-running Andy Carpenter series set in Paterson, New Jersey.