House of Suns

Six million years ago, at the dawn of the human starfaring era, Abigail Gentian split herself into a thousand clones and launched them into the galaxy, to gather more memories and wisdom than one single human being could ever accumulate in a universe bound by Einstein's laws. Periodically the shatterlings of Gentian Line meet for a grand bacchanalian reunion, where, over the course of a thousand heady nights, they exchange memories.
Two wayward shatterlings, Campion and Purslane, are about to be decades late for Gentian Line's thirty-second reunion. Even worse, they have fallen in love ; the only thing standing between them and possible excommunication from the Line is the golden robot Hesperus, one of the Machine People, the other great meta-civilisation occupying the Milky Way. But Hesperus is an amnesiac, and all he can recall is a vague sense that he was on a mission of desperate importance before his memory was wiped.
The lovers are hoping that providing a prestigious guest will mitigate their punishment - until an unexpected distress signal makes censure by their fellow clones the least of their worries. After six million years of stability someone has decided it is time to end gentian Line.