The distinguishing trait of this novel is its structure. It's a story within a story, within a story. The framing device features characters that are familiar to many, and takes place between Wizard and Glass, and Wolves of the Calla. If you're unfamiliar with the Dark Tower series, this won't mean anything to you, but it really has no bearing on the story anyway. It's a stand alone novel that doesn't have much to do with what came before, or after in the series, so if you've haven't read the whole series - or any of it for that matter - you aren't missing anything.
The Ka-tet of Roland, Susanna, Jake and Eddie, are caught up in a fearsome, once in a lifetime storm, and to pass the time, Roland tells them a tale of his youth.
Sent to investigate tales of a "skin man" terrorizing a small town at the edges of civilization, the young Roland finds the only survivor and witness to a massacre is a young boy. To calm the frightened child he tells the boy a story about another boy who embarks on a quest.
Tim Ross lives with his parents at the edge of the Endless Forest. When his father dies under mysterious circumstances a dark and mystical stranger appears to offer answers. Tim embarks on a life changing journey into the depths of the deadly Endless Forest. This third tale makes up the bulk of the novel.
The Wind Through the Keyhole is a surprisingly quick read. Because of the many problems I had with the latter half of the series (if you've read all seven books, you probably know what I'm talking about), I was afraid that the story, particularly with this type of nesting doll structure, would get out of hand. I quickly found that I didn't have anything to fear. I had a lot of trouble putting it down, even when I knew I only had five hours before I had to wake up (and I'm useless without a full eight hours). Many of the "quirks" that would mark a story out as a Stephen King novel are missing, making it a refreshing change.
In short, I would recommend The Wind Through the Keyhole to anyone within shouting distance.