We Were Liars, by E. Lockhart

Each summer, the Sinclair family gathers on their private island. This story is about these reunions, especially during the summer that Cadence, the protagonist, is unable to remember.  Cadence suffers a mysterious accident and spends the next two years—and the course of the book—in a haze of amnesia, migraines, and painkillers, trying to piece together just what happened. This book kept me engaged.  I couldn't put it down.