Novel by H. Beam Piper based on the short story "Graveyard of Dreams", published in Galaxy Magazine February 1958. Originally published as Junkyard Planet, but most editions published as "The Cosmic Computer".
During the System States' War, Poictesme? was the general HQ and supply depot for the final thrust at the enemy. When the war ended, the buildings, the munitions, the freeze-dried food supplies, were all abandoned without a thought. Now the colony world is a poverty-stricken agricultural society with only two exports: the fermented products of their world's unique grapes, and the salvaged war equipment, now selling at about 1% of its true value.
And, persisting over the decades, is the legend of MERLIN, the super-computer said to have planned the grand strategy which successfully concluded the war. "If we could only find Merlin," the inhabitants said to each other, "all our problems would be solved."
Then young Conn Maxwell returned from Earth, with a university degree, and a few clues about the location and the true nature of Merlin. And the sure knowledge that finding the Cosmic Computer would be the worst thing that could possibly happen to his home world.