Greg Bear

City at the End of Time Greg Bear Reprint edition

In a time like the present, in a world that may or may not be our own, three young people?Ginny, Jack, and Daniel?dream of a decadent, doomed city of the distant future: the Kalpa. But more than dreams link these three: They are fate-shifters, born with the ability to skip across the surface of the fifth dimension, inhabiting alternate versions of themselves. And each guards an object whose origin and purpose are unknown: gnarled, stony artifacts called sum-runners that persist unchanged through all versions of time.

Hunted by others with similar powers who seek the sum-runners on behalf of a terrifying, goddess-like entity known as the Chalk Princess, Ginny, Jack, and Daniel are drawn into an all but hopeless mission to rescue the future?and complete the greatest achievement in human history.