Jack of Shadows is a 1971 book by Roger Zelazny. Jack is a Darksider, born on the dark side of a tidally locked planet. Lightsiders are human: they grow old, they die, they have no magical powers. Darksiders on the other hand are reborn after death and have magical powers. Jack is also a thief, and the reader's first impression of him is "charming rogue," but after he's been killed and reborn (not a pleasant process), he dreams of brutal revenge. Having broken the social compact on the Darkside, he's forced to flee to the Lightside - he's one of the very few Darksiders capable of this, as most Darksiders have to stay in their "place of power." There he uses science - and a computer - to try to change the balance of power on the Darkside.
You can take some hints from the name of his only friend: Morningstar, a gigantic gargoyle-like creature moored to a mountain until he sees sunrise as a punishment (on a tidally locked world where the Darkside never faces the sun, that's a very long time). And Jack is pursued for much of the book by "the Borshin," a hideous creature made by a Darksider interested in science who was trying to create life through magic. "Abortion" anyone?
Jack is initially an appealing character, but he becomes less and less so as the book progresses. The magical world where he initially resides is kind of interesting, but it becomes pretty unpleasant as people try to catch and kill him or torture him for all eternity and he pursues his own revenges. What drives both magic and science on this strange world remain almost entirely unexplained - and what Jack learns that tips the balance of power so spectacularly is totally unexplained. I like explanations and I like sympathetic characters. The book was kind of interesting, but lacked both so I'm not much of a fan.