Joe Haldeman & Roger Zelazny 
Jack of Shadows 

In a world half of light, half of darkness, where science and magic strive for dominance, there dwells a magical being who is friendly with neither side. Jack, of the realm of shadows, is a thief who is unjustly punished. So he embarks on a vendetta. He wanders through strange realms, encountering witches, vampires, and, finally, his worst enemy: the Lord of Bats. He consults his friend Morningstar, a great dark angel. He is pursued by a monstrous creature called the Borshin. But to reveal any more would be to spoil some of the mindboggling surprises Jack of Shadows has in store. First published in 1971 and long out-of-print, Jack of Shadows is one of fantasy master Roger Zelazny’s most profound and mysterious books.