The Lavalite World
by Philip José Farmer
Urthona's private world was a place of sudden land shifts...of mountains that sank into valleys, and plans that rose into peaks. Kickaha was trapped there with Anana and they were all in desperate straits. Because there was only one gate out of that world, located within Urthona's palace. And finding that palace, which moved as often as the landscape itself, was by no means a simple task.