A Void in Hearts - William G. Tapply

Boston lawyer Brady Coyne investigates the death of a shady private detective in a mystery with "complications of the heart as compelling as clues" (Publishers Weekly).
Les Katz may well be scum. A private detective, he does not hesitate to take the case when a Farrah Fawcett look-alike hires him to tail her husband. The photos he secures suggest the man is cheating on his wife, but they aren't definitive. Rather than disappoint his client, he contacts her man and offers to sell him the pictures. Katz considers this a charitable act, but to his attorney, Brady Coyne, it looks an awful lot like blackmail.
Brady tells Katz to give the money back, fully expecting to be ignored. But when Katz is killed in a hit-and-run, he realizes blackmail wasn't the PI's only mistake: Les Katz was murdered to protect a terrible secret—and a conspiracy that goes far beyond a single cheating husband.