Donna Tartt The Little Friend Book

Alexandria, Mississippi. On Mother's Day, a little boy named Robin was found hanged in his own yard.
Twelve years later, this murder remains unsolved, and the family has never recovered from the tragedy. The mother fell into a depression, and the father fled to another city. So, as she grows up, Harriet — a nervously bright, unbearably determined sister of Robin — decides to find his killer.
Relying on the help of her devoted friend Hely, Harriet begins the search for the culprit. Very soon, the investigation ceases to be a game...