Eye of the Needle Ken Follett

One enemy spy knows the secret to the Allies' greatest deception, a brilliant aristocrat and ruthless assassin -- code name: "The Needle" -- who holds the key to ultimate Nazi victory.
Only one person stands in his way: a lonely Englishwoman on an isolated island, who is beginning to love the killer who has mysteriously entered her life.
All will come to a terrifying conclusion in Ken Follett's unsurpassed and unforgettable masterwork of suspense, intrigue, and the dangerous machinations of the human heart.