What is the main idea for the entire book

Daniel begins the book at 14 years old. point of view as he experiences the atrocities of the holocaust for Jews in central Europe. When the story opens, he is 14 years old. Daniel is riding on a train and looking at pictures in his photo album. Daniel uses a photograph album to spark his memory of events from when he was six years old until the day he and his family are loaded onto the train bound for the Lodz ghetto.