Quest Crosstime — Andre Norton

The universe had opened up to Blake Walker when he discovered that his world -- our own Earth -- was just one of hundreds ranked side by side on alternate timelines, each more strange and wondrous than the last, all coexisting at once, and all available to Blake Walker and the few natives of those other-Earths who could break the barriers between timelines.

But Blake's new existence is threatened now -- his time-travelling patron's daughter has been kidnapped. The engineers who could control the time-travel process are in open rebellion. And Blake himself has stumbled across a plot by a powerful and dangerous band of conspirators to ravage all of the Earths in their reach.

Blake must stop them. He knows that somewhere in the infinity of time there lies the key to solving this crisis. If he only knew when to look....