"God Save the Child" by Robert B. Parker (Spenser series, Book 2)

A boy is gone, the victim of a kidnapping or his parents' indifference, Spenser couldn't tell. Either way, Spenser has been hired to find him, and now that meant going up against a sexual predator masquerading as a hero. 

But into this ugly case comes a beautiful woman named Susan Silverman, who wanted to save a child, and maybe a detective, too...