"What do we owe the past? How to make peace with a dark family history? Burkhard Bilger hardly knew his grandfather growing up. His parents immigrated to Oklahoma from Germany after World War II, and though his mother was an historian, she rarely talked about her father or what he did during the war. Then one day a packet of letters arrived from Germany, yellowing with age, and a secret history began to unfold. Karl Gönner was a schoolteacher and...