Charles Sheffield - Between the Strokes of Night

After the Nuclear Spasm in the 21st century, homo sapiens was extinct, save for a tiny remnant scattered in small, primitive space colonies. At first Solar Humanity had only one goal: survival. But when the battle for existence was won, humankind began moving outward in slow, multi-generation space ships, and as then millennia passed, planet-based civilisations emerged in many star systems. In the year 27,698 A.D, to these new worlds come the Immortals, beings with strange ties to ancient Earth, who seem to live forever, who can travel light years in days - and who use their strange powers to control the existence of ordinary mortals. On the planet Pentecost, a small group sets out to find and challenge the Immortals. But in the search they themselves are changed: as Immortals, they discover a new threat, not just to themselves, but to the galaxy itself.