What so impressed me on that first reading was the self-containedness of Tolkien's world. I suppose there are a few novelists who have created worlds that are uniquely their own -- Faulkner, for example, or Dickens. But since their world is fairly close to the actual world, it cannot really be called a unique creation. The only parallel that occurs to me is the Wagner Ring cycle, that one can only enter as if taking a holiday on a strange planet.