Dealing with multiple worlds for world building is usually more the purview of science fiction than it is the purview of fantasy literature. It is almost a given in any science fiction novel that several different planets will be visited. Each one of these worlds must be built to make the novel an enjoyable read. It is rarer to deal with more than one planet in fantasy literature, unless it is a kind of split-off parallel world existing in the same space as ours as a kind of fairy land. Usually, in fantasy literature, there is no need for multiple worlds (planets) in different locations in space, particularly since transportation between such worlds becomes problematic. One must normally rely on magic to get from planet A to planet B. And once you do that, it is difficult to see how planet B will be understood as anything but a split-off parallel world much like the aforementioned fairy land. As a result of that, fantasy world building is usually limited to one planet and multiple dimensions, because in an odd sort of way, different dimensions actually seem closer to us than different planets—at least from the point of view of fantasy literature.
Still, for every rule there is an exception. A previous commenter on this blog pointed out to me that Brandon Sanderson is setting all his novels in the same universe. Each world he writes about exists in the same dimension. He even has a character, a story-teller/bard/what-have-you by the name of Hoid who shows up in each one of his books. I don’t know where he’s going with that, but it is kind of cool. Particularly since he deals with religious themes, with gods ruling humans, sometimes dying, and evil always on the rise. I suspect Hoid is an agent of, or perhaps even the avatar of, the one Supreme Being of the whole cosmos. A kind of god’s God. But that is Brandon Sanderson, and he’s the first writer I’ve heard of to connect his novels in such a way. And it is worth pointing out that as far as each individual fantasy novel is concerned (so far), it takes place only on the world in which it is set. So far, none of his The Way of Kings characters intermix with his Mistborn characters or anything like that. At least, not yet. Although Sanderson’s idea connects all his work into a single inter-related body, it can only take him so far. He might write well and fast, but the universe is an awfully big place with billions of galaxies, let alone who knows how many planets. Although his idea is original, clever, and genuinely really cool, he can only carry it through so far. There is just too much out there to encapsulate his vision fully. Maybe he’ll write about a dozen or two dozen worlds in his career. Two dozen worlds does not a universe make. The scope is just too vast to be manageable. Still, Sanderson is a brilliant writer and he may pull it off in ways I can’t foresee.
If you take anything away from this post, it should be the importance of scope. World building, be it a single world, multiple dimensions, or even an entire universe, is always a limited project. The real world parallels to it are always far more vast and complicated than the writer’s ultimate accomplishment. As I said, there’s just too much there to fully encapsulate. The trick is to give enough to give an impression of completeness without overwhelming the reader.