Double Deuce by Robert B. Parker

Spenser is forced by loyalty into an alien world where violence is a way of lie and outsiders enter at a lethal risk. When Spenser's cohort, Hawk, is hired by the tenants of a gang-plagued Boston housing project known as "Double Deuce", he enlists his friend's aid.
A friend's girl and her infant daughter have been gunned down. Though the act at first appears to be an accidental drive-by shooting, it soon becomes clear that it was premeditated murder. Before they can solve the crime, Spenser and Hawk must take on an adolescent band of hardened urban warriors. As bullets fly and the brutality escalates, Spenser learns more than he ever dreamed about a generation imprisoned in a hell of poverty and hopelessness where muscle is the ticket to survival, and the surest way out is in a body bag.
Pulsing with moral complexity, Double Deuce is the kind of no-holds-barred action thriller only Robert B. Parker can create.