Warning: Possible spoilers below:

The Gunslinger is the first book in the Dark Tower series by Stephen King. He tells the story of Roland, a gunslinger from the inner baronies (Gilead actually), who embarks on a quest to find the Dark Tower. He decides on this quest after finding a piece of the Wizard's Rainbow in Hambry (see Wizard and Glass).
During Roland's quest, as he chases the Man in Black, he encounters Jake, a young boy from New York who finds himself in the Gunslinger's world after being pushed in front of a car and dying. Roland and Jake set out after the Man in Black together until Jake is killed once again, this time by the Gunslinger himself. Jake's final words to Roland are "Go then, there are other worlds than these".
Roland continues west and eventually meets up with the Man in Black who, during a long palaver, foretells that Roland will draw three companions to help him on his continuing journey toward the Dark Tower.