Philip E. High 
Prodigal Sun 

Power-Greedy.Men wished to rule the world, but one man stood in their way. Peter Duncan came from the planet Mattrain – but who was he? Was he human? If so, whence came his vastly superior intellect and technical knowledge? These were mysteries the Administration feared – because they could not find the answer to them. Too late they saw danger. For, by then, Peter Duncan had escaped and taken refuge in the Devastated Areas, from which he continued his fight to save the human race from final utter destruction. But then, he had a reason. Martha of the chestnut hair and striking beauty, who alone knew his secret and who taught him how to love.