The setting for Roger Zelazny's Chronicles of Amber.
"The series is based on the concept of parallel worlds, domination over them being fought between the kingdoms at the extreme ends of Shadow - Amber, the one true world of Order, and the Courts of Chaos. Amberites of royal blood - those descended from Oberon (and ultimately his parents: Dworkin, formerly of the Courts of Chaos, and the Unicorn of Order herself) - are able to "walk in Shadow", mentally willing changes to occur around them. These changes are, in effect, representative of the Shadow-walker passing through different realities. There are apparently infinite realities, and the characters in the novels are not sure if these different universes are created as one walks through Shadow, or if they already exist and a Shadow-walker is able to slip from one to another. In the Merlin cycle there are references to the Wheeler-Everret interpretation of quantum-mechanics and the Ghostwheel created by Merlin is said to "shuffle" through Shadows, suggesting that the multiverse exists independently, although this is never explicitly stated.
Within this multiverse, Zelazny deals with some interesting philosophical concepts about the nature of existence, compares and contrasts the ideas of Order and Chaos, and plays with the laws of physics - they can differ from Shadow to Shadow; for instance, gunpowder does not ignite in Amber, which is why the characters all carry swords. Other Shadows have green skies and blue suns, cities of glass and Kentucki Fried Lizzard Partes, and worlds out of our own fiction can come to life."
Source: Wikipedia, "The Chronicles of Amber", available under the CC-BY-SA License.