The setup of this (the fourth book in the Gunslinger series) is a framing story for Roland's earlier life. This book covers the time in Roland's life immediately after he becomes a gunslinger as a young teenager. 
 One thing I really liked about this story is it's a return to the more concrete storytelling of the first book, with well-developed characters and settings. I felt in particular the third book veered off into Stephen King's crazy preordained quest territory too much (see for example The Stand or It). Those sorts of stories are fun to read as well, but get a little tiring because the storytelling is so linear and predictable. In comparison, I really did not know exactly where this novel would go (other than towards some sort of horrible and depressing conclusion of course).