The Invisible Harry Gold: The Man Who Gave the Soviets the Atom Bomb by Allen M. Hornblum.
Even with the overstated title, this book caught my eye because of recent discussion about Solzhenitsyn's In the First Circle. More on Harry Gold can be found here.
From the Yale University Press site linked above:
In the history of Soviet espionage in America, few people figure more crucially than Harry Gold. A Russian Jewish immigrant who spied for the Soviets from 1935 until 1950, Gold was an accomplished industrial and military espionage agent. He was assigned to be physicist Klaus Fuchs’s “handler” and ultimately conveyed sheaves of stolen information about the Manhattan Project from Los Alamos to Russian agents. He is literally the man who gave the USSR the plans for the atom bomb. The subject of the most intensive public manhunt in the history of the FBI, Gold was arrested in May 1950. His confession revealed scores of contacts, and his testimony in the trial of the Rosenbergs proved pivotal. Yet among his co-workers, fellow prisoners at Lewisburg Penitentiary, and even those in the FBI, Gold earned respect, admiration, and affection.