The Professional — Robert B. Parker

A knock on Spenser's office door can only mean one thing: a new case. This time the visitor is a local lawyer with an interesting story. Elizabeth Shaw specializes in wills and trusts at the Boston law firm of Shaw & Cartwright, and over the years she's developed a friendship with wives of very wealthy men. However, these rich wives have a mutual secret: they've all had an affair with a man named Gary Eisenhower- and now he's blackmailing them for money. Shaw hires Spenser to make Eisenhower "cease and desist," so to speak, but when women start turning up dead, Spenser's assignment goes from blackmail to murder.

As matters become more complicated, Spenser's longtime love, Susan, begins offering some input by analyzing Eisenhower's behavior patterns in hopes of opening up a new avenue of investigation. It seems that not all of Gary's women are rich. So if he's not using them for blackmail, then what is his purpose? Spenser switches tactics to focus on the husbands, only to find that innocence and guilt may be two sides of the same coin.