The Huntress by Kate Quinn 

A gripping thriller about the hunt for a woman, who as a Nazi killer hunted children.  The tale is told in two timelines; one about a Russian girl who wanted to fly airplanes and the other, after World War II where she and her ‘husband’ look for the Nazi killer.  The story exposes the bravery of Russian women who flew planes and bombed the enemy Germans to defend and then to drive back the invaders.  And the dangerous escape of one such woman after the government chose to imprison her over something her father said in a drunken brawl.  She then rescues an escaped British soldier, only for him to get killed by the huntress, who tried to kill them both.  But the Russian woman escaped and survived.  Later, the soldier’s brother, a journalist, finds her in a field hospital and to save her, he marries her in the field and has her sent to England.  A few years later, after the war is over, he has been chasing down Nazi criminals to bring them to justice and she appears to help him find the Huntress (who has gone to America).