All the Broken Places by John Boyne 

Ninety-one-year-old Gretel Fernsby has lived in the same well-to-do mansion block in London for decades. She lives a quiet, comfortable life, despite her deeply disturbing, dark past. She doesn’t talk about her escape from Nazi Germany at age twelve. She doesn’t talk about the grim postwar years in France with her mother. Most of all, she doesn’t talk about her father, who was the commandant of one of the Reich’s most notorious extermination camps.

Then, a new family moves into the apartment below her. In spite of herself, Gretel can’t help but begin a friendship with the little boy, Henry, though his presence brings back memories she would rather forget. One night, she witnesses a disturbing, violent argument between Henry’s beautiful mother and his arrogant father, one that threatens Gretel’s hard-won, self-contained existence.