The Visitor by Sheri S. Tepper

The 'magic' that once was America died horribly along with most of the Earth's inhabitants when an asteroid crashed into the planet sometime during the twenty-first century. Hundreds of years have passed, and all that remains of the time before are fragmented memories distorted by superstition -- as a tragically reduced populace suffers greatly under the tyranny of a repressive ruling order. But destiny has chosen Dismé Latimer to lead a wasted world out of the darkness ... with a book. Written by a courageous scientist ancestor, it is a sacred, unsettling tome rife with disturbing ideas and revelations ... and an impossible hope that compels a gentle, troubled young woman to abandon her abusive home in search of truth and her true self. But common 'wisdom' and lore warn of grave dangers out in the world. Evil is there, a malevolence beyond imagining. And in the depths of the Earth, a gargantuan beast asleep for centuries has begun to stir ...

Note: This image is from a stock photo and may differ from the actual cover.