The Woman with the Blue Star / Pam Jenoff.

"From the New York Times bestselling author of The Lost Girls of Paris comes a riveting tale of unfathomable sacrifice and unlikely friendship during World War II. 1942. Sadie Gault is eighteen and living with her parents amid the horrors of the Kraków Ghetto during World War II. When the Nazis liquidate the ghetto, Sadie and her pregnant mother are forced to seek refuge in the perilous sewers beneath the city. One day Sadie looks up through a grate and sees a girl about her own age buying flowers. Ella Stepanek is an affluent Polish girl living a life of relative ease with her stepmother, who has developed close alliances with the occupying Germans. Scorned by her friends and longing for her fiancé, who has gone off to war, Ella wanders Kraków restlessly. While on an errand in the market, she catches a glimpse of something moving beneath a grate in the street. Upon closer inspection, she realizes it's a girl hiding. Ella begins to aid Sadie and the two become close, but as the dangers of the war worsen, their lives are set on a collision course that will test them in the face of overwhelming odds. Inspired by harrowing true stories, The Woman with the Blue Star is an emotional testament to the power of friendship and the extraordinary strength of the human will to survive"--