You know me; I love to stretch definitions to the breaking point. In this case, though, I won't even attempt to argue that L. Frank Baum's 1900 tale, The Wonderful Wizard of Oz, is a pulp fantasy book, because even I'm not addled enough to make that claim. That said, I do think this children's novel is an important one for our hobby, for reasons I'll now elucidate.
I often say that one of the reasons Dungeons & Dragons has been misunderstood by many gamers (and designers) is because they misunderstand its cultural roots. In the late 60s and early 70s, fantasy and science fiction weren't as diverse as they are today. Fans of those genres -- which were generally considered the same genre rather than distinct ones -- all read the same books and authors, creating the common literary culture out of which D&D arose. That's not to say that there wasn't a lot of variation from fan to fan and from place to place, but the underlying foundations for those fandoms showed a high degree of commonality. Thus, allusions to certain characters, situations, and authors were all broadly
understood rather than being opaque or outright impenetrable. The death of that common culture has, in my opinion, led to the distancing of D&D from its roots to the point of unrecognizability.
The story of Baum's novel is another example of a common culture. I suspect most people in the English-speaking world -- certainly in the United States at any rate -- know the story and characters of The Wonderful Wizard of Oz. When I was a child, in the benighted days before VCRs and cable television, watching the 1939 Victor Fleming movie was an annual ritual, broadcast each year as a network TV special. People could and still do reference this story with the expectation that just about everyone knows what they mean when they do so. That's what many fantasy and SF stories were like at the dawn of this hobby and it was that kind of ready familiarity that allowed the ideas contained in those three little brown books to catch fire and spread so easily.
There's a catch, of course. When I say that everyone "knows" the story and characters of Baum's novel, that's not entirely truthful. The reality is that the 1939 film changes many elements of the original, just as earlier dramatic productions of the story did, often with Baum's blessing, who regularly wrought his own changes through sequels to the initial book. In Oz fandom -- yes, such a thing does exist -- there are discussions and debates every bit as vociferous as those in this hobby regarding the merits of such changes, not to mention continuations and re-interpretations by later authors. The Oz books are all in the public domain and have been for some time, allowing the original ideas to be picked up and developed further by any author who wishes to do so, leading to a wide variety of non-Baum Oz tales, some of which stray very far from the original intent -- not unlike D&D itself.
The Wonderful Wizard of Oz is, if nothing else, a powerful example of fantasy world building. The Land of Oz and its inhabitants have no doubt exercised a powerful influence over the imaginations of many a participant in this hobby, including some of its earliest writers. Oz is a "pure" fantasy world -- a dreamland (though not literally so in the novels, where it's clearly a real place) that obeys its own laws and confounds expectations. It's thus about as far from a naturalistic world as you can get and yet it's drawn so colorfully that one forgets its inherent unreality and simply accepts it for what it is. This approach to fantasy was much more common in the hobby's early days, as anyone who reads the 'zines of the period will know. That's probably why you can find lots of references to Oz-inspired adventures, items, and monsters in their pages. A full treatment of the Land of Oz as a D&D campaign setting was, in fact, promised in the pages of Dragon for many years but never saw print so far as I am aware.
I don't think there's any hiding the fact that I miss the days when the hobby had a stronger common culture -- when one could allude to "The Tower of the Elephant" or "Ill Met in Lankhmar" and not be met with blank stares, when we could cite Manly Wade Wellman or Henry Kuttner and people would not only know to whom you referred but had actually read their fiction. To the extent that such a common culture exists anymore, it comes from the ouroboros of D&D itself, a substitute for the books and authors that once formed the Common Tongue by which gamers conversed. I'm not sure such days can ever return -- the hobby is simply too diverse now -- but I long for it just the same.