Who? is a fine novel by Algis Budrys, now also lost to us.
I only met Budrys once, at my first Worldcon in 1987. He was a judge for the Writers of the Future contest, and I mentioned to him that the glossy certificates handed out to the runners-up (I'd just won my first), while pretty, didn't tell us what we'd done right or wrong. After that, the stories I sent to the contest always came back with a few lines of helpful feedback.
As editor of Tomorrow, he treated writers with courtesy and was always prompt to reply (I used to say that Tomorrow and Century were the only magazines named after their response times, though that was a slight exaggeration. Budrys was fast, but his rejections still took the usual two weeks to arrive by airmail). The world is poorer now that he's gone.
Who? was last reprinted in 2000, and there are still some new (or as-new) copies listed on bookfinder.com, so it's not as lost as say, Margaret Tabor's Eclipse, which deals with a similar theme in a very different way (and which I'll discuss in a later column). Who? tells the story of Lucas Martino, an American physicist badly injured in a laboratory explosion while doing weapons research. Russian medics reach him before the Americans do, and the man they had over has a metal head (with a human brain inside) and a prosthetic arm. Dental records, retinal scans, voiceprints... none of these can be relied on, and DNA profiling had not yet been discovered. The cyborg's only identifying features are the fingerprints on one arm, which has obviously been sewn on - but is it re-attached, or transplanted? Is this man Martino, or is he a Russian infiltrator who's been very well briefed on Martino's life?
The cyborg attempts to prove that he is, indeed, Martino, but the Americans are understandably reluctant to let him near a lab again, or even to ask him questions about Martino's work that might tell the Soviets too much. An agent is assigned to discover who the cyborg really is, and finds the task incredibly frustrating. It's a good blend of cold war espionage story, mystery novel, and science fiction, which occasionally touches on the question of what is it that makes us who we are.