Foundation's Edge

Foundation's Edge is a novel by Isaac Asimov, the fourth book in the Foundation Series. It was written thirty years after the Foundation trilogy, due to pressure by fans on Asimov to write another, and, according to Asimov himself, the amount of the payment offered by the publisher.

Warning: Wikipedia contains spoilers

Plot

The Mayor of Terminus fears that the mysterious Second Foundation, which is thought to be extinct, will bid for power once the new Galactic Empire is established. Councilman Golan Trevize and Janov Pelorat, Professor of Ancient History at the University of Terminus are sent on a mission to discover its location.