The Risen Empire (Succession, 1) [Westerfeld, Scott]

Captain Laurent Zai and his pacifist lover Senator Nara Oxham, separated by lightyears, hold the fate of the empire in their hands. They serve an Emperor who has reigned with his sister the Child Empress for sixteen hundred years as living gods.

But even gods might bow to the fanaticism of the Rix, machine-augmented humans, who long for the cybernetic rule of a vast AI.

Only one immortal dynasty can prevail.