I love Laura Lippman's mystery series starring Tess Monaghan, but she has written several stand alone novels.
What the Dead Know is one of them. A middle aged woman leaves the scene of a car accident, and when she's questioned by the police, she refuses to reveal her identity. But she hints that she is one of a pair of sisters who disappeared thirty years ago. But if she is one of the long lost Bethany sisters, why has she been gone so long? Where has she been? And where is her sister? Lippman has won the Edgar as well as the Shamus, Agatha, Anthony and Nero Wolfe awards, and she is always a pleasure to read.