The Reader by Bernhard Schlink

For 15-year-old Michael Berg, a chance meeting with an older woman leads to far more than he ever imagined. The woman in question is Hanna, and before long they embark on a passionate, clandestine love affair which leaves Michael both euphoric and confused. For Hanna is not all she seems. Years later, as a law student observing a trial in Germany, Michael is shocked to realize that the person in the dock is Hanna. The woman he had loved is a criminal. Much about her behaviour during the trial does not make sense. But then suddenly, and terribly, it does - Hanna is not only obliged to answer for a horrible crime, she is also desperately concealing an even deeper secret.