I don't know enough science to explain the planet where Jack lives. It doesn't rotate, so half is always in light (where science and technology are common) and half is always in darkness (where magic holds sway). There's a shadowy border area, and that's where Jack thrives. He has no powers in complete light or darkness, but he draws it from shadow. He's one of the few who can travel between the worlds. As the book opens, he's on the light side, where he's about to steal something. He's a thief, after all. He's caught and executed, but darksiders have serial immortality, and so does Jack. He's tossed into the Dung Pits of Glyve, where he regenerates and gradually makes his way back to the darkside, where he plots his revenge, along with a rescue.
Jack's name (so said Zelazny at one time) is a bow toward Jack Vance, and the "travel through marvels" half of the book (the first half) is Vance-like in some ways. I was reminded a bit of the stories in The Dying Earth, and Jack isn't entirely unlike Cudgel the Clever. The second half is different and darker, and I've heard more than one person express dislike for the book because of it. I liked it, though, and I have nothing against the open ending. I probably thought when I read the book back in the early '70s that Zelazny would write a sequel. I might even have hoped he would. But he never did.
That's okay, though. There's still this one, and it was fun to visit it again.