Book excerpt: What if you could not only travel any location in the world, but to any possible world? We can all imagine such “other worlds”—be they worlds just slightly different than our own or worlds full of magic and wonder—but it is only in fiction that we can travel to them. From The Wizard of Oz to The Dark Tower, from Philip Pullman's The Golden Compass to C. S. Lewis's The Chronicles of Narnia, there is a rich tradition of this kind of fiction, but never before have the best parallel world stories and portal fantasies been collected in a single volume—until now.