Before anybody jumps in with "The Marching Morons," I want to defend Carrel a little here: He was no more on board with eugenics than most of the educated elite in the 1930s. A good book to put the eugenics movement into perspective is War Against the Weak by Edwin Black. That'll give you chills if nothing else will; the Nazis did not spontaneously erupt from a vacuum.

Man the Unknown is well worth reading, though there's a slightly weird quality to it today, as though it came from an alternate universe. (You can get copies from ABEBooks for as little as $3 or $4.) The notion of radically improving the body through technology is one that was probably mentioned earlier, but which I think Carrel put on the map.