What the Dead Know by Laura Lippman
A middle-aged woman causes an accident on the Baltimore Beltway, flees the scene and is later picked up wandering on the shoulder of the Interstate. The accident occurs just a mile from the former home of the Bethany family, Dave and Miriam and their daughters Sunny and Heather. Thirty years before, twelve-year-old Heather persuaded her older sister Sunny to let her tag along on a visit to the mall. Neither of the girls has been seen since. Now, the woman on the Beltway claims to be Heather Bethany. Today she has a different name; a different identity. What has prompted this woman to announce her true - if it is true - identity at this moment? With her customary acute intelligence and empathy, Laura Lippman deftly unpeels the layers of Heather's past: revealing her parents, sister, friends, teachers, and the detective who handled the case, until the truth about what happened to Heather and Sunny on that long ago afternoon is finally uncovered.