Fair warning: Spoilers ahead.
The Waste Lands begins with signs that Roland Deschain, the gunslinger is slowly going mad. At the end of the previous book, he stopped the Pusher from shoving Jake (the boy who appears in the first book) in front of a car, thus preventing events from the first book from ever happening. This creates an interesting temporal paradox, in which the gunslinger begins to experience split realities — one in which Jake dies and one in which he never met Jake. As time goes on, his mind becomes more and more divided between these two realities.