Sweet Tooth by Ian McEwan

The year is 1972. The Cold War is far from over. Britain is being torn apart by industrial unrest and terrorism. Serena Frome, in her final year at Cambridge, is being groomed for MI5. Serena is sent on a secret mission – Operation Sweet Tooth – which brings her into the world of Tom Haley, a promising young writer. First she loves his stories, then she begins to love the man. Can she maintain the fiction of her undercover life? And who is inventing whom? To answer these questions, Serena must abandon the first rule of espionage – trust no one.

Eye of the Needle by Ken Follett

His weapon is the stiletto, his codename: The Needle. He is Hitler's prize undercover agent - a cold and professional killer. It is 1944 and weeks before D-Day. The Allies are disguising their invasion plans with a phoney armada of ships and planes. Their plan would be ruined if an enemy agent found out... and then The Needle does just that. Hunted by MI5, he leads a murderous trail across Britain to a waiting U-Boat. But he hasn't planned for a storm-battered island, and the remarkable young woman who lives there . . .