V is for Vengeance by Sue Grafton

From Library Journal: "Kinsey Millhone doesn't look the other way when she sees trouble, so when she spots a woman shoplifting, she immediately informs store authorities. This sets off a chain of events, as the woman is soon found dead of an apparent suicide. Her fiance doesn't believe she killed herself, and Kinsey's quest to find the truth puts her on the trail of a major shoplifting ring. Grafton's latest alphabetical mystery brings Kinsey into contact with a number of shady characters, from gangsters and gamblers to unhappy and unfaithful spouses. Grafton's pioneering sleuth is as clever and witty as ever."