Charles Sheffield 
Between the Strokes of Night [EPUB ebook] 

In the 277th century, Earth is dead, but mankind survives in colonies scattered across the galaxy. 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.