Michael Morpurgo writes so simply but with so much impact. The story is nominally about a young journalist who has an interview with a famous violinist. The journalist is told not to ask him the “Mozart question”. Of course, she does somehow come around to it and thus begins the story of how musical inmates of certain concentration camps were made to play classical music (and especially Mozart) as trains of new inmates came in to the camps.
While these musical inmates were better fed and treated, it left them with terrible guilt which they continued to live with even after the war. Young readers would need to have some background information about the Holocaust to understand the full meaning of the book.