stephenking.com says:

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.

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Gaaaah, The Talisman! It must be 25 years since I last read this. I remember liking this *a lot* and spending several lunch breaks discussing it with my friend Tina.
I wonder how much my perception will have changed now that I am no longer the hero's age but more like that of the authors' at the time. ;p