The Brain-Stealers

From the first page of the Ace Double: "He alone defied the cosmic vampires! When the outlawed scientist Jim Hunt leaped from the prison plane, he had no suspicion that he was not the only one falling silently through the midnight sky. But other, stranger exiles were landing at the very moment in the same backwoods region... exiles from the unknown depths of outer space, exiles seeking human food. When Jim started to make his way home, he discovered the full horror of that night's events. For the people he met had become mere flesh-and-blood puppets, mindless creatures going the bidding of the unseen invaders. And though every man's hand was against him, both free and enslaved, Jim knew that he alone was humanity's only hope for survival."