The Lavalite World — Philip Jose Farmer

Urthona's private world was a place of sudden land shifts...of mountains that sank into valleys, and plains that rose into peaks. Kickaha was trapped there with Anana and three other reluctant companions -- and they were 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.