Who? by Algis Budrys (1958)
An American nuclear scientist is injured in an explosion near the Iron Curtain. A Soviet team responds and carries him over the border for medical attention. When he is released four months later, Allied intelligence are presented with a man with a stainless-steel skull, an artificial arm and a chest filled with mechanical organs. Is this Dr Lucas Martino? A Soviet agent? Who?
On the surface, Who? is a Cold War espionage novel revolving around the mystery of who the man with the metal head is. How, in the absence of the usual tests can his identity be established? He has no face, no teeth to match against dental records and his one remaining arm is certainly Martino's, but is the rest of the body his? How can you even question a man who doesn't react properly because his organs are now artificial? Worse, how far can security conciousness go before it becomes counter-productive? How much doubt is reasonable doubt? This works as a lightweight thriller and there's even a coy twist when we learn that the KGB mastermind is as equally worried about his Western counterpart as the other way around. However, Who? is a much deeper novel as it parallels the problem of who the mystery man is against the life of Dr Martino. In flashbacks, we see the maturing of a young genius as he follows a carefully thought-out career that leaves him little room for a personal life. As the story progresses, it soon becomes apparent that part of the reason this mystery exists is that before his accident Martino was already something of a cipher; a man as dedicated to his work as a thinking machine whose lack of friends and family make it so easy for him to vanish inside his steel shell.
Part spy story and part study about a man being forced to rediscover the life that he turned his back on and attempt to reconnect with a world he'd cut himself off from, Who? is another example of the sort of small sci fi stories that don't get written today.