Robert B. Parker's lullaby

From Amazon: When fourteen-year-old Mattie Sullivan asks Spenser to look into her mother’s murder, he’s not convinced by her claim that the wrong man was convicted. Mattie is street-smart, wise beyond her years, and now left to care for her younger siblings and an alcoholic grandmother in a dilapidated apartment in South Boston. But her need for closure and her determination to make things right hits Spenser where he lives. As Spenser becomes more involved, he thinks that Mattie may be onto something after all. And he’s going to need the help of his friend Hawk to find peace for Mattie—a job that’s more dangerous than he ever thought.