memoirs of a teenage amnesiac (Farrar, Stauss and Girioux, 2007) by Gabrielle Zevin is very, very good. It's an interesting concept, and very believable.
16-year-old Naomi fell and hit her head. She can't remember anything from the last 4 years: her boyfriend; her parent's divorce; her half sister; her best friend, Will Landsman, who calls her Chief; why she loves working on the Yearbook; French; how to drive a car. She doesn't know why she was with Ace; she's interested in James, the new guy, who rescued her when she fell. She decides she doesn't like who she was. THEN, she remembers . . . everything!
Go to author Gabrielle Zevin's website to read about all the awards this book has garnered. There's also a book trailer, and news that it's being made into a movie!
Gabrielle also wrote Elsewhere (review here) and a number of books for adults.