Margaret Cavendish, Duchess of Newcastle, wrote The Blazing World (1666), a book far ahead of its time, in which the heroine passes through a portal near the North Pole to a world with different stars in the sky and talking animals.
Edwin A. Abbott, mathematician and theologian, wrote Flatland (1886), also known as Flatland: A Romance of Many Dimensions. It recounts the story of a two-dimensional world inhabited by living geometric figures: triangles, squares, circles, etc., and explores concepts of other dimensions (or universes) including Pointland, Lineland, and Spaceland. A feature film adaptation of this novella was made in 2007 called Flatland The Film.
Murray Leinster's story "Sidewise in Time" (1933), showing different parts of the Earth somehow occupied by different parallel universes, was influential in science fiction.
H. Beam Piper, the author of the Paratime series, wrote several stories dealing with alternate realities based on points of divergence far in the past. The stories are usually written from the perspective of a law-enforcement outfit from a parallel reality which is charged to protect the secret of temporal transposition.
Fredric Brown's What Mad Universe recounts the adventures of a science-fiction editor of the late 1940s who is thrown into a parallel universe that reflects the fantasies of his most annoying......