The Reader by Bernhard Schlink

When he falls ill on his way home from school, fifteen-year-old Michael Berg is rescued by Hanna, a woman twice his age. In time she becomes his lover—then she inexplicably disappears. When Michael next sees her, he is a young law student, and she is on trial for a hideous crime. As he watches her refuse to defend her innocence, Michael gradually realizes that Hanna may be guarding a secret she considers more shameful than murder.