The Risen Empire — Scott Westerfeld

The undead Emperor has ruled his mighty interstellar empire of eighty human worlds for sixteen hundred years. Because he can grant a form of eternal life, creating an elite known as the Risen, his power has been absolute. He is worshiped as a living god. No one can touch him.

Not until the Rix, machine-augmented humans who worship Al minds of planetary extent. The Rix are cool, relentless fanatics, whose only goal is to propagate such Als throughout the galaxy. They seek to end the Emperor's tyranny, and replace it with an eternal cybernetic dynasty. They begin by taking hostage the Child Empress, his sister, who is eternally a little girl. Captain Laurent Zai of the Imperial Frigate Lynx is tasked with her rescue.

Separated by light years, bound by an unlikely love, Zai and pacifist Senator Nara Oxham must each, in their own way, face the challenge of the Rix, as they hold the fate of the empire in their hands.