City of Illusions — Ursula K. Le Guin

Earth, like the rest of the Known Worlds, has fallen to the Shing. Scattered here and there, small groups of humans live in a state of semi-barbarism. They have lost the skills, science and knowledge that had been Earth's in the golden age of the League of Worlds, and whenever a colony of humans tries to rekindle the embers of a half-forgotten technology, the Shing, with their strange, mindlying power, crush them out. There is one man who can stand against the malign Shing, but he is an alien with amber eyes and must first prove to paranoid humanity that he himself is not a creature of the Shing.