Foundation's Edge, written by Isaac Asimov, is the fourth book in the original Foundation series. Published in 1982, it was written over thirty years after the original trilogy and became Asimov's first novel to make the New York Times best-seller list. 
 
Foundation's Edge is set roughly 500 years after Hari Seldon established the Foundation to preserve human knowledge during the fall of the Galactic Empire. 
 
The First Foundation believes it has defeated the mythical Second Foundation and is on track to become the new galactic empire.
However, councilman Golan Trevize suspects that the Second Foundation may still exist and that Hari Seldon's plan is being manipulated. He is exiled for his political views but sent on a mission with historian Janov Pelorat to search for Earth, the legendary planet of human origin.
Meanwhile, the current leader of the Second Foundation also senses that the plan is being affected by an external force.
The dual plotlines converge on the planet of Gaia, where the protagonists are confronted with a new, larger factor at play in the fate of the galaxy