The Talisman - Stephen King

Twelve-year-old Jack Sawyer embarks on an epic quest--a walk from the seacoast of New Hampshire to the California coast--to find the talisman that will save his dying mother's life.

Jack's journey takes him into the Territories, a parallel medieval universe, where most people from his own universe have analogs called "twinners." The queen of the Territories, Jack's mother's twinner, is also dying.