When one looks at the list of authors Gary Gygax assembled in Appendix N, chances are good that most of the names at least are familiar, even if one hasn't read the particular stories associated with them. They're a veritable who's who of pre-1970s fantasy and science fiction, writers whose works formed the canon of these genres. A notable exception is Margaret St. Clair, one of only two women on the list -- the other being Leigh Brackett -- and one whose name was utterly unknown to me, never mind her novels.
When I was a younger man, I had no trouble getting hold of books by Howard or Merritt or Leiber, so I devoured those and promptly forgot about St. Clair and her 1969 novel The Shadow People. It wasn't until years later that I actually came across a copy of it in a used book store and picked it up on a whim. I dimly recalled the name and figured it might be worth reading if Gary considered it an inspiration on AD&D.
I'm glad I did so, because, whatever its shortcomings as a novel, it's one that (I think) gives some insight into the origin of everyone's favorite dark elves, the drow. The Shadow People is about human beings stumbling upon the dark fairy world that exist just behind the walls of certain places in the world, where twisted, emaciated elves lay in wait, plotting the downfall of the world above. Addicted to hallucinogenic fungi, their plots often go awry thanks to their penchant for treachery against their own kind, a trait that has probably saved humanity far more than outright heroics against the dark elves.
It's hard not to see the drow in St. Clair's dark elves, but perhaps I'm simply projecting them backward in time. After all, part of the drow's appeal is how archetypal they are. They draw so brilliantly on centuries of myths and legends about dark elves while having their own unique spin that they seem exactly as our subconscious would imagine them to be. St. Clair's dark elves have similar qualities, though they are a bit more explicitly mythical than are the drow, right down to their use of the hallucinogenic fungi to lure human beings into their own surreal kingdom.
I can certainly see why Gygax found the book so attractive. He had a great love for creepy fairy realms, something that reached its fullest flower in his post-D&D games, Mythus and Lejendary Adventure. The Shadow People is well written and enjoyable as a story in its own right, but I found it particularly useful for its presentation of another take on dark elves. I found myself with plenty of great ideas to swipe for my Dwimmermount campaign and I suspect I won't be the only referee mining St. Clair's novel in this way.