The book is set in a world whose rotational period is equal to its revolutionary period. Thus one side of the planet is always in light, and the other in darkness. Science rules on the dayside, while magic holds sway in the night.
Shadowjack, the main character, is unique among the magical beings in that he draws his power not from a physical location but from shadow itself. He is nearly incapacitated in complete light or complete darkness, but given access to even a small area of shadow, his potency is unmatched.
The plot consists of multiple threads interacting on multiple levels, as is common in Zelazny's writing. One theme is Jack's own development. He begins as a soulless demon ("Jack of Evil"), bent upon conquering his enemies and taking all he wants. As he faces various challanges, we witness a conscience and deep maturity begin to emerge. Jack's only friend, the creature Morningstar, is trapped in stone at the edge of the night; his torso and head protrude from the rock, and he awaits the sun that will never rise. Morningstar's role is beautifully interwoven into the story -- his conversation with Jack is a masterpiece of dialog in fiction.
Jack also seeks the Key That Was Lost, Kolwynia. The Key itself and the consequences of its use parallel Jack's progress in his own endeavors. Ultimately, the Key will be responsible for Jack's salvation and his doom.
After completing this relatively short novel, the reader is struck by a profound sense both of completion and bewilderment, in the perfect proportions of which Zelazny is a master. Highly recommended.