Number the Stars by Lois Lowry
Ten-year-old Annemarie Johansen and her best friend Ellen Rosen often think about life before the war. But it's now 1943 and their life in Copenhagen is filled with school, food shortages, and the Nazi soldiers marching in their town. The Nazis won't stop. The Jews of Denmark are being "relocated", so Ellen moves in with the Johansens and pretends to be part of the family. Then Annemarie is asked to go on a dangerous mission. Somehow she must find the strength and courage to save her best friend's life. There's no turning back now.