Book 3 of The Tales of Alvin Maker.

From the 18th century into the early years of the 19th, Americans—traveling to find new homes and new lands—crossed the Appalachian Mountains and moved across the Northwest Territory, spreading west to the banks of the great river. But in this alternate frontier America, the men and women use lore and folk-magic to settle the continent. When Alvin arrives in Hatrack River to take up his apprenticeship with Makepeace Smith and learn to be a blacksmith, he finds that nothing is as he expected. This would-be maker is on his own and the work of the Unmaker is close at hand.