The Professional, by Robert B. Parker

Typical Spenser, in that he gets the job done, although in this case his employers try to call him off and he actually ends up liking the 'bad guy' a little more than the people who hired him. The case centers on a con man who really enjoys sleeping with women who are married to wealthy older men. After setting the hook, he then blackmails them but somehow still manages to charm them into thinking he really loves them. And then...things go badly wrong. And Spenser fixes it. Sort of.