Seeker, now a member of the Questioners intergalactic peacekeeping force, encounters the winged Vyinnlyirr, who have built a beautiful city on the plundered ruins of a planet, while forcing the native Others to live outside in a smoking wasteland.
From Wikipedia: Dennis Arthur Schmidt was an American science fiction and fantasy author publishing from 1978 to 1990. Common threads in his books are religion, discipline and mystical enemies.