The Reader by Bernhard Schlink

This book tells the story of Michael, a fifteen year old who becomes involved with a much older woman. He quickly falls in love with her and puts up with her strange temper tantrums because he's afraid of losing her. Then one day she disappears without any explanation. He doesn't see or hear from her again until he happens upon a trial where she is inexplicably refusing to defend herself against horrible crimes she is accused of committing as a concentration camp guard. Michael tries to reconcile the love he still feels for her with the idea that she was involved in the killing of Jews.