Eye of the Needle
Ken Follett

Eye of the Needle
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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.

Synopsis from GoodReads
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