This Island Earth — Raymond F. Jones

A purchasing agent accidentally intercepts technology far beyond human science, leading radio engineer Cal Meacham to construct an “interocitor” -- a device that connects him to the enigmatic Peace Engineers. Recruited to their secretive desert compound, Cal is drawn into a shadowy struggle involving spaceships, mind control, and galactic warfare. As he uncovers their true mission, he must decide whether to support their covert battle or expose their perilous secrets. In a race against deception and cosmic enemies, the fate of Earth could rest in one man's judgment.