The Foundation Trilogy by Isaac Asimov

It is the story of the Galactic Empire, crumbling after twelve thousand years of rule. And it is the particular story of psycho-historian Hari Seldon, the only man who can see the horrors the future has in store: a dark age of ignorance, barbarism and violence that will last for thirty thousand years. Gathering together a band of courageous men and women, Seldon leads them to a hidden location at the edge of the galaxy where he hopes they can preserve human knowledge and wisdom against all who would destroy them.

Asimov went on to add numerous sequels and prequels to the trilogy, building up what has become known as the Foundation series, but it is the original three books, first published in the Forties and Fifties, which remain the most powerful, imaginative and breathtaking.