There's been a lot of talk in the old school community, especially this blog, about the thief class introduced in Supplement I to OD&D. In discussing it, you'll see frequent references to the Gray Mouser, Cugel the Clever, and even Conan, but comparatively few to Roger Zelazny's Jack of Shadows, who makes his appearance in the 1971 novel of the same name. I kick myself for not having remembered Jack, all the moreso because Gary Gygax was always quite upfront about taking inspiration from this character when imagining the thief class. Indeed, Appendix N lists Jack of Shadows (along with the "Amber" series, with which it bears some thematic similarities) as one of the books that inspired AD&D.
I suppose it's because I'm not actually a big fan of Zelazny's writing that I'd forgotten this vital clue to "The Thief Question." There's something a bit too "psychedelic" about Zelazny's style that doesn't appeal to me. My tastes are much more staid in comparison. Still, that's no excuse for having overlooked Jack of Shadows, which, despite its unevenness, full of fascinating ideas. The title character hails from a world that does not rotate. One side, dominated by magic, is perpetually in darkness, while the other side, dominated by science, is perpetually in light. Jack, as you might expect, exists in the shadows, his powers negated in either complete darkness or complete light. Though a magical being with more in common with the creatures of darkness, Jack nevertheless straddles the two worlds and the novel's story is about his attempts to gain mastery over both, culminating in a series of events that forever change the world -- and Jack himself.
Jack of Shadows is a good example of the kind of quirky world-building for which the fantasy genre was once known. The setting of the novel is neither a post-Tolkien epic fantasy nor a grim and gritty Robert E. Howard pastiche. Instead, it's a purely fantastical place, where the laws of reality vary from place to place and magic and science reign supreme within their confined realms. In short, it's an original exercise of the imagination and, while I have my issues with both its presentation and content, I can't help but approve of its uniqueness.
For me, though, the most important part of Jack of Shadows is the insight it offers into the thief class and its abilities. Jack is a magical being, a master of manipulating the realm between shadow and light to his advantage. He is the greatest thief that ever was, because he is at one with the shadows. The D&D thief wasn't intended to emulate Jack, so I don't think it's reasonable to assume that its class abilities are "magical" in nature. Nevertheless, that influence is there and I think it needs to be borne in mind when considering what the thief is supposed to be and how its abilities are supposed to work.
There's always a danger in focusing too heavily on a single influence over any aspect of D&D, of course, but I nevertheless think it's vital to understand all those influences. Without that knowledge, one might mistakenly change or remove something ignorant of the consequences. To some extent, I may be guilty of that with regard to the thief, as I recently admitted. That's why I'm glad I remembered Jack of Shadows. I'm still uneasy about the thief and its effects both on OD&D specifically and the development of Dungeons & Dragons generally, but now I have some additional influences to consider as I ponder these matters.