In a way, I'm surprised that early D&D didn't take more from the Oz novels - its pure American fantasy. It's also a fully-fleshed out campaign setting if you take all the books together. Then again, in another way, I'm not surprised, as it would clash pretty badly thematically with the war gaming and sword-and-sorcery roots of the early years of the game; I'd imagine that it would be considered too "childish". However, these days, it would be perfect as a Domain of Delight in the Feywild, especially as much of the novel series are now public domain. I'm really surprised that no one (at least as I'm aware of) hasn't leapt on this and published it as a campaign setting.