The World as Myth

Robert A. Heinlein's The Number of the Beast introduces the concept of the "World as Myth" which supposes that all fictional universes are equally real and, moreover, are accessible to one another via interdimensional travel.

The act of authorship is what creates said universes, which leads to the interesting notion that the characters in any given universe may be controlled, at any given moment, by an Author from another. Characters could, in theory, meet their own Author. Conversely, each Author is a character in someone else's fiction.

The Number of the Beast and its two sequels make extensive use of the concept to permit a Massively Multiplayer Crossover between all the fictional universes that Heinlein wrote and those of several other authors of his era.

Heinlein novels explicitly set in the World as Myth:

 * The Number of the Beast
 * The Cat Who Walks Through Walls
 * To Sail Beyond the Sunset

Tropes embodied or directly implied by the World as Myth:

 * Alternate Universe
 * Cosmic Retcon
 * Massively Multiplayer Crossover
 * The Multiverse
 * Mutually Fictional
 * Rage Against the Author
 * Recursive Canon
 * Time Travel