The Talisman by Stephen King

The Talisman is a 1984 fantasy novel by Peter Straub and Stehen King. The plot is not a reworking of the earlier Walter Scott book also titled The Talisman, although there is one oblique reference to "a Sir Walter Scott novel." The Talisman was nominated for both the Locus and World Fantasy Awards in 1985. Stephen King and Peter Straub followed up in 2001 with a sequel, Black House, that picks up with a now-adult Jack as a retired Los Angeles homicide detective trying to solve a series of murders in the small town of French Landing, Wisconsin. As Peter Straub stated in December 2009, he and Stephen King should begin work on a third novel in the series sometime in late 2010.
The book The Talisman is dedicated to the authors' mothers: "This book is for Ruth King, Elvena Straub." The book The Talisman charts the adventure of a twelve year old boy named Jack Sawyer. The adolescent hero sets out from Arcadia Beach, New Hampshire in a bid to save his mother, who is dying from cancer, by finding a crystal called "the Talisman."
The premise of the novel The Talisman involves the existence of a parallel world to Earth, called "the Territories", a strange fantasy world with ties (though these ties did not exist at the time of publication) to Stephen King's The Dark Tower. Individuals in the Territories have "twinners," or parallel individuals, in our world. Twinners' births, deaths, and (it is intimated) other major life events are usually paralleled. Twinners can also flip, but only share the body of their alternate universe's analogue. (Summary by Wikipedia)