Seventh Son - the first "Alvin Maker" novel by Orson Scott Card. This was a Hugo-award nominee in 1988. The first 5 chapters appeared in Isaac Asimov's August 1986 as "Hatrack River." I have read most of books in this series and enjoyed them, but sometimes had problems with believability. Not so with this one - this lives up to any hype you may have heard. It is a marvelous example of world-building and alternate history extrapolation - demonstrating Card's range by branching into Fantasy after having won the Hugo and Nebula awards for best novel in SF - in the previous 2 successive years.
I read my copy a long time ago, so here's the flap copy from a hardcover:
From the end of the 18th century into the early years of the 19th, Americans crossed the Appalachian Mountains and moved across the Northwest Territory, spreading west to the banks of the great river. They traveled to find new homes, new lands, and they brought with them the plain magics of plain people...
Using the lore and folk magic of the men and women who settled a continent, and the beliefs of the tribes who were here before them, Card has created an alternate frontier America; a world where a particular kind of magic really works and where that magic has colored the entire history of the colonies. Charms and beseechings, hexes and potions, all have a place in the lives of the people of this world. "Knacks" abound: dowsers find water, sparks set fires, blacksmiths speak to their iron, the second sight warns of dangers to come, and a torch can read the heart-fire of anyone withing reach. It is into this world, in a roadhouse on the track westward, amid the deep wood where the Red man still holds sway, that a very special child is born. Young Alvin is the seventh son of a seventh son, born while his six brothers all still lived. Such a birth is powerful magic; such a boy is destined to become something great - perhaps even a Maker. But no Maker has been born for many a century, and there is no lore to tell how the Maker's knack works. At the age of six Alvin doesn't seem to have any special talents at all, unless it's the knack he has of working with stone and wood, crafting tools and ornaments; unless it's the way he has with animals... Yes, Alvin is something special; and even in the loving safety of his home, dark forces reach out to destroy him. Something will do anything to keep Alvin from growing up.