The Quest for Anna Klein by Thomas H Cook
One freezing night in 1939, Thomas Danforth accepts a mission that will change his life forever: travel to Nazi Germany as cover for an enigmatic female assassin, Anna Klein.
Danforth is captivated by this beautiful, mysterious woman. But just as he starts to discover more about her, she disappears...
Desperately searching for her through a Europe ravaged by war, Danforth becomes ensnared in a maelstrom of love and vengeance, of shifting loyalties and betrayals. His quest will span decades, implicate him in a world-wide conspiracy, and take him to the brink of madness. But he cannot shake the obsession that drives his every waking moment: Who is Anna Klein?