The Chalk Girl by Carol O’Connell

The
little girl appeared in Central Park: red-haired, blue-eyed, smiling,
perfect—except for the blood on her shoulder. There is something
about the girl that Mallory understands. Mallory is damaged, they
say, but she can tell a kindred spirit. And this one will lead her to
a story of extraordinary crimes: murder, blackmail, and a particular
cruelty that only someone with Mallory’s history could fully
recognize.