The Secret Galactics

Reality twisted... slightly. Earth shivered. For a split second the Solar System wasn't. And then was again. Less than a billionth of a second - but a time shift had nonetheless happened.
As the shadow ship started to emerge from the time jump, men and aliens were locked in a secret, undeclared war for control of the Earth. Genetically altered, the aliens looked exactly like humans. And they were in positions of power everywhere. Opposing them were two humans and one isolated brain in a mechanical body. But between them they held the one secret that the aliens had never learned about the people of Earth....
It was also published by DAW Books as "Earth Factor X". It is partly based upon the short story "The Sound of Wild Laughter".