Jack of Shadows by Roger Zelazny.

Jack is a darksider whose power comes from shadows. He is a thief who is out for revenge on his executors. You follow him from the Dung Pits of Glyve to his part in the fate of the world.

The world consists of the light side and dark side. The light side is much like our world, with science and technology. The dark side is run by magic, and dark lords. Jack travels between both, poignantly stopping to visit Morningstar who is perputually trapped at the Gates of Dawn.

Zelazny likes survivors, especially those with a goal. The goal may not be noble, but it does have to be all-driving. Jack's goal starts off as revenge, but changes to something grander.

I had heard good things about of this book for many years. I don't understand why publishers give Zelazny's books such a hard time: They publish very short runs if they publish them at all! Luckily, I managed to obtain a copy of Jack of Shadows. Shorter than I expected, but definitely worth it.