I'm about a third of the way through 'The Storyteller' by Jodi Picoult. It's about a young reclusive girl with a scarred face who befriends an elderly man who is very respectable, pillar of the community type, who confides to her that he was a Nazi during WWII.
It's pretty harrowing stuff, reading his account of how he became a Nazi and the things he did, but from the reviews I think the 'worst' is still to come, with the story of the main characters grandmother who was a prisoner in Auschwitz.
That said, and as hard as it is to read, I am hooked and can't put it down.