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:
Tropes embodied or directly implied by the World as Myth: