Interstellar Empire — John Brunner

Once there was a mighty empire spanning the stars. Once there had been great emperors ruling hundreds of inhabited planets, patrolling the star lanes with fleets of starcruisers, built to the perfection of a technology never before seen by man.

And like all empires in human history, that one, the greatest of them all, declined and fell. Its colony worlds were left to themselves, falling into a galactic Dark Age of ignorance, barbarism, superstition. But they had one heritage in common - the imperial starships still functioned. they were self-repairing, foolproof, impervious, and obeyed the commands of any who reached their simplified controls.