The Risen Empire
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-after-death, creating an elite known as the Risen, his power is absolute. He and his sister, the Child Empress, who is eternally a little girl, are worshipped as living gods.

The Rix are machine-augmented humans who worship very different gods: AI compound minds of planetary size. Cool, relentless fanatics, their only goal is to propagate such AIs. They seek to end the Emperor’s prolonged rule, and supplant it with an eternal cybernetic dynasty. They begin by taking the Child Empress hostage. 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 both face the challenge of the Rix, and both will hold the fate of the empire in their hands. 

*From the paperback*
Imperialism in fiction, Immortalism in fiction, Fiction, Imperialism, Hostages
Scott Westerfeld