THE SILENT INVADERS
The first American single-book appearance.
He looked like a human: Major Abner Harris, born in Ohio. But he was really Aar Kiilom, a disguised Darruui of the highest class, a servant of the spirit, who had no right to question his orders. His orders said the universe was in danger of enslavement by the pebble-skinned Medlins. And that the fight against their evil schemes called for him to assume the role of Abner Harris, Earthman. But once in that synthetic body, he made a mind-numbing discovery: that the real villians of space were neither the Medlins nor the people of Earth, but his own kind! Suddenly he was alone, alienated from his own race, hated by the Medlins, and an impostor on Earth - no matter which side he chose, he'd be a traitor - but choose he must or forever remain a man without a planet...
Expanded from magazine version in Infinity October 1958, as by "Calvin M. Knox." The first book appearance was as half of a 1963 Ace Double (as by Silverberg.)