Vault of the Ages — Poul Anderson

This classic from science fiction's Golden Age envisions a primitive future where only the forbidden technology of the past can save an endangered tribe from violent extinction

Five centuries into the future, America is no longer what it used to be. Many generations after the old civilization was consumed in a nuclear firestorm, the few survivors have returned to a simple, agrarian lifestyle and eschew the malevolent “magic” of science that brought about “the Doom.” When the Dalesmen of the Alleghenies are threatened by the hostile Lann descending from the North, their chieftain's son, Carl, is dispatched to the blasted ruins of an ancient city to trade for metal and weapons.
 
However, something extraordinary awaits him in this feared and forbidden place: a time capsule from the twentieth century containing the essential scientific and technological information of a lost society. Carl knows his discovery can help rescue the Dalesmen's homes and farms. But the law is unbending in this age of superstition -- and if the chieftain's son returns this forbidden knowledge to his people, the tribal Doctors will surely put him to death.

One of Poul Anderson's earliest science fiction masterworks, Vault of the Ages is at once a riveting tale of adventure and a deeply human story of duty and destiny offering a stunning vision of an all-too-possible future.