Second Foundation (Foundation #3) by Isaac Asimov

After years of struggle, the Foundation lay in ruins–destroyed by the mutant mind power of the Mule. But it was rumored that there was a Second Foundation hidden somewhere at the end of the Galaxy, established to preserve the knowledge of mankind through the long centuries of barbarism. The Mule had failed to find it the first time–but now he was certain he knew where it lay.

The fate of the Foundation rests on young Arkady Darell, only fourteen years old and burdened with a terrible secret. As its scientists girded for a final showdown with the Mule, the survivors of the First Foundation began their desperate search. They too wanted the Second Foundation destroyed… before it destroyed them.