The Magician’s Nephew by CS Lewis
The Narnia series is a classic of children’s fiction for a reason, but it’s this lesser-known entry that most embraces the idea of multiverses. The first chronologically in the series, The Magician’s Nephew you introduce the world that serves as connecting tissue between others (including the “real world” and Narnia). It’s a heady entry, concerned largely with Lewis’ very Christian ideas of creation and the role of humanity in larger cosmology. Weird enough to deserve its spot as an intriguing read for adults and operating under a quaint brand of fairy tale logic that made it well-loved among children, it’s been a worthy part of the multiverse conversation since 1955.
The Golden Compass by Phillip Pullman
Another children’s classic, the series that starts with The Golden Compass (and recently got a BBC adaptation as His Dark Materials) is often in conversation with Narnia. While the second book deals more directly with alternate universes, the first is essential reading. In short, it’s a world where humans have external manifestations of their souls in the form of animals.
In the first entry, we get a sense of the stakes of alternate cosmology as a resource, and the corrupt church trying to use its power for their own evil ends. Later books introduce more alternate earths and a chase between dimensions. It’s also fun fantasy on its own, with the animal-like daemons as one of the most original and compelling ideas in the genre to this day.
Wayward Children series by Seanan McGuire
Every Heart a Doorway, the first book in this novella series, is as much in conversation with Narnia as the Pullman books are. Instead of tackling the religious element, though, it focuses on the emotional impact. What happens to people when they come back through the doorway? Her characters from Ella have a wide spectrum of relationships with their worlds beyond the portals and the fantasies they played out there.
Each novella introduces a different world and theme, but some characters recur throughout. The difference universes are typically standalone: you won’t find alternate selves here, or time travel. But it’s still a world of new views opening up at every turn, and of characters exploring what that means for their relationships to each other.
The Eyre Affair by Jasper Fforde
Alternate selves, metafiction and time travel all swirl around in this absurd series filled with literary in-jokes. Detective Thursday Next is our viewpoint character in this wildly detailed alternate world. Starting out in an English society obsessed with literature and high-tech enough to have time travel and cloning, it bounces into metafiction with multiple versions of fictional characters and alternate Thursdays. This one delivers more puns and hijinks than deep musings about the nature of everything, but the inventive world-building makes it memorable and entertaining. The series began in 2001, with the latest entry in 2012.