Robert B. Parker's Someone to Watch Over Me

In the latest Spenser thriller, the legendary Boston PI and his young protégé Mattie Sullivan take on a billionaire money manager running a network of underaged girls for his rich and powerful clients
Ten years ago, Spenser helped a teenage girl named Mattie Sullivan find her mother's killer and take down an infamous Southie crime boss. Now Mattie - a college student with a side job working for the tough but tender PI - dreams of being an investigator herself. Her first big case involves a fifteen-year-old girl assaulted by a much older man at one of Boston's most prestigious private clubs. The girl, Chloe Turner, only wants the safe return of her laptop and backpack. But like Spenser, Mattie has a knack for asking the right questions of the wrong people.
Soon the two find ties between the exploitation of dozens of other working class girls to an eccentric billionaire and his sadistic henchwoman with a mansion on Commonwealth Avenue. The mystery man's wealth, power and connections extend well beyond Massachusetts - and maybe even the United States. Spenser and trusted ally Hawk must again watch out for Mattie as she unravels a massive sex-trafficking ring that will take them from Boston to Boca Raton to the Bahamas, crossing paths with local toughs, a highly-trained security company, and Spenser's old enemy the Gray Man for a final epic showdown.