Atonement by Ian McEwan
On the hottest day of the summer of 1934, thirteen-year-old Briony Tallis sees her sister Cecilia strip off her clothes and plunge into the fountain in the garden of their country house. Watching her is Robbie Turner, her childhood friend who, like Cecilia, has recently come down from Cambridge. By the end of that day, the lives of all three will have been changed for ever. Robbie and Cecilia will have crossed a boundary they had not even imagined at its start, and will have become victims of the younger girl's imagination. Briony will have witnessed mysteries, and committed a crime for which she will spend the rest of her life trying to atone.