All That I Am by Anna Funder

When Hitler seizes power in 1933, a tight-knit group of friends and lovers suddenly become hunted outlaws overnight. Dora, liberated and fearless; her lover, the great playwright Ernst Toller; Ruth; and Ruth's journalist husband, Hans find refuge in London. There, using secret contacts deep inside the Nazi regime, they take breathtaking risks to warn the world of Hitler's plans for war. But England is not the safe haven they think it will be, and a single, chilling act of betrayal will tear them apart....

Based on true events, All That I Am is testament to some of the earliest--now forgotten--heroes of the resistance to Hitler.