The Snowfall Trilogy

The Trappers, hunter-gatherers who live beside a mammoth wall of ice, have none of the wonders their carefully hand-copied, ancient Warm Times books describe, like cars, guns, or plows. When bloodthirsty invaders slaughter most of the tribe, a small band of survivors heads south, led by a man few of them trust. Holding the remnants of her people together is their doctor, Catania Olsen, whose ugliness has left her unmarried but whose medical skill has earned her people's respect.