Who? (1958) is a science fiction novel by American writer Algis Budrys, set during the Cold War.
Plot according to wikipedia:
An explosion resulting from an experiment gone awry rocks an Allied research facility near the border with the Soviet Bloc. A Soviet team abducts Dr. Lucas Martino, a leading Allied physicist in charge of a secret, high-priority project called K-88.
Several months later, under American pressure, the Soviet officials finally hand over an individual, claiming that he is Dr. Martino. The man has undergone extensive surgery for his injuries. He has a mechanical arm that is more advanced than any produced in the West. More importantly, his head is now a nearly featureless metal skull, a kind of extreme craniofacial prosthesis. A medical evaluation reveals that several of the man's internal organs are also artificial. His biological arm and its hand's fingerprints are identified as Martino's, but this may be the result of an arm and hand transplantation. The Allies are suspicious that the Soviets have sent them a technologically altered spy and are holding the real Martino for further interrogation.