Ursula K. LeGuin
City of Illusions
Scattered over the earth, small isolated settlements of humanity lived in a state of semi-barbarity; the knowledge and skills, the glory that had been Terra's in the golden age of the League of Worlds lost to ruthless conquerors.
Each time a group of Earthmen began to rise fom the ashes, the Shing, the ravagers of Terra, the Liars, would crush them out.
Only one man could stand against them, and that man had alien amber eyes. Before he dared oppose the enemy he had to prove his humanity, and to himself that he was not a tool of the Shing....