As further evidence for my thesis that L. Sprague de Camp and Fletcher Pratt are important keys to understanding Gary Gygax's vision for Dungeons & Dragons, there's their 1948 collaboration, The Carnelian Cube. Like many of the titles listed in Appendix N, this novel is a story of alternate realities and of a modern man transported between them. This is a such a common theme in Gygax's list of "inspirational and educational reading" that I can't help but wonder why no published D&D product, either during his time at TSR or subsequently, dealt with it, except very obliquely. The notion of a person from contemporary society flung into another world has a long pedigree and contains rich veins of adventure to mine. One wonders why it never seems to have had been an appreciable impact on the development of D&D, despite Gary's repeated references to books dealing with precisely this theme.
The Carnelian Cube itself tells the tale of an archeologist named Arthur Finch who confiscates a weird stone -- the eponymous carnelian cube -- from one of the workers at his dig in Turkey. The stone, it turns out, belonged to the Pythagorean philosopher Apollonius of Tyana, to whom later generations attributed various occult powers. When Finch places the stone under his pillow and goes to sleep dreaming of a more perfect, "rational" world, he awakens to find himself in a parallel Earth, where everything is done on a solely rational basis, resulting in the elevation of self-interest above all else. Finch wishes to escape but, unfortunately, the carnelian cube does not make its way with him and he must seek out the parallel version of the worker from whom he confiscated the stone so he can return home. Regain the cube he does, but with unintended effects, for each time he steals it and dreams, he finds himself transported into yet another parallel world that isn't quite what he wanted.
The Carnelian Cube may not have had any specific influence over D&D, but I can see its influence over the World of Greyawk, as Oerth is one of a series of parallel worlds, each one containing versions of the same people and places but subtly different, owing to the unique nature of each parallel. Likewise, if you look at Gary Gygax's work as a whole, you can see plenty of examples where he treated the topic of alternate worlds and realities and the possibilities for adventure therein. One might be able to argue that D&D's portrayal of the multiverse owes something to Gary's enjoyment of alternate world tales. Like most things in the game, the application of its inspirations is often quirky and not immediately obvious. That probably is the case here too, but I am nevertheless left wondering what Dungeons & Dragons -- and the wider hobby -- might be like today if stories like The Carnelian Cube had exercised a more clear influence.