The Golden Age of science fiction (1940s-1960s) produced many excellent stories about parallel worlds. Robert Silverberg handled the theme particularly well. I recall one about a traveler from parallel world A (Alexander the Great lived to a ripe old age and Hellenized the world) visiting parallel world B (a North America that had been colonized in the tenth century by Magyars and Vikings). Beautifully done, with a nice twist at the end that would appeal to Andrew Sullivan. Can’t remember the name. Readers?