The fourth book in King's Dark Tower series is a "prequel" of sorts. It tells a story about Roland that occurs long before we meet him in The Gunslinger. I discussed this with someone who thought that this book is not a Dark Tower book. I disagree. Despite the change in the setting, it may actually be my second favourite book in the series, right after The Gunslinger. However, I am not yet sure how the story in this book continues the plot of the series as a whole.