Number the Stars, by Lois Lowry, 1989

It's 1943, and 10-year old Annemarie Johansen and her family have lived under the grip of the Nazi occupation of Denmark for two years. She's almost grown used to the soldiers on every corner, the shortages of clothing, butter, coffee, and the silent, secret tension shared by all the adults she knows. But when the Nazis begin rounding up Copenhagen's Jews for "relocation," placing her best friend Ellen Rosen in danger, Annemarie learns firsthand what bravery means. Told with strength, simplicity, and restraint, tonally consistent with the Danish heroism to which it is a tribute. Strongly recommended.