What the Dead Know by Laura Lippman.
This is an assumed-identity novel in the vein of Mary Stewart's The Ivy Tree and Josephine Tey's Brat Farrar. Yes, this is as well-written and absorbing as those two classics.
The story begins when a young woman is involved in a car accident while driving into Baltimore. Although she flees the scene, she is found by police a little while later. But she has no ID, and when the police question her, she admits to being one of "the Bethany girls", two sisters who had been kidnapped 30 years earlier, and had never been found.
Is she really Heather Bethany? The circuitous path to the answer is absorbing and believable. I stayed up way too late finishing this book last night, and it was worth it!