Lilac Girls by Martha Hall Kelly

New York socialite Caroline Ferriday has her hands full with her post at the French consulate and a new love on the horizon. But Caroline’s world is forever changed when Hitler’s army invades Poland in September 1939—and then sets its sights on France. An ocean away from Caroline, Kasia Kuzmerick, a Polish teenager, senses her carefree youth disappearing as she is drawn deeper into her role as courier for the underground resistance movement. In a tense atmosphere of watchful eyes and suspecting neighbors, one false move can have dire consequences. For the ambitious young German doctor, Herta Oberheuser, an ad for a government medical position seems her ticket out of a desolate life. Once hired, though, she finds herself trapped in a male-dominated realm of Nazi secrets and power. The lives of these three women are set on a collision course when the unthinkable happens and Kasia is sent to Ravensbrück…