Barnes & NobleThe Butterfly by Patricia Polacco

Ever since the Nazis marched into Monique's small French village, terrorizing it, nothing surprises her, until the night Monique encounters 'the little ghost' sitting at the end of her bed. She turns out to be a girl named Sevrine, who has been hiding from the Nazis in Monique's basement. Playing after dark, the two become friends, until, in a terrifying moment, they are discovered, sending both of their families into a nighttime flight.