A Hostage for Hinterland
Ballantine Books 
Synopsis: A thousand years after the last Limited Nuclear War [Number 13, I think], two cultures divide North America between them. The Structures are highly limited in area but stretch many kilometers into the air, held up by antigravity devices, whose cooling is dependent on helium imported from the Hinterlands. The Hinterlands are occupied by numerous tribes, both hostile to technology and dependent on it, especially the high tech which is primarily only available from the Structures. Every five years, both sides hammer out trade agreements and as the book opens, the current set of negotiations are about to begin.