It has been eight years since Stephen King presented readers with the controversially open ending to his epic fantasy sequence, The Dark Tower. The seven-book series eventually stretched to nigh-on 4,500 pages, and it would have been fair to assume the author was done with a story he began over three decades ago, inspired by Robert Browning's poem "Childe Roland to the Dark Tower Came", and by the spaghetti westerns of the 1970s.

But Roland the gunslinger and his battered band of followers  a drug addict, a boy, a schizophrenic woman missing half her legs  and their quest for the Tower have clearly exerted more of a pull over King than any of his other creations; he even, somewhat bizarrely, wrote himself in to later parts of the story. So perhaps it's not surprising that he has found a way back to Mid-World, a place which is "very old, and falling to ruin, filled with monsters and untrustworthy magic", and to the characters who just won't stay quiet.

The Wind Through the Keyhole is set in between the fourth and fifth novels in the sequence, making it, King says, Dark Tower 4.5. Roland and his followers have left the Emerald City, but have yet to arrive at Calla Bryn Sturgis; they are forced to hole up in an abandoned meeting hall when a storm of monstrous proportions, the starkblast, hits them. But their adventures only top and tail the novel, because The Wind Through the Keyhole is a story within a story within a story, as Roland talks his way through a long, icy night.

First, the gunslinger tells his friends of a mission in his youth to save a desert town from a "skin man", drawing them in with the hard-to-beat opener: "Not long after the death of my mother, which as you know came by my own hand  " This shapeshifter is suitably, horrifically murderous, leaving a trail of blood and guts in his wake. Just one survivor remains from the last attack, a young boy, and to keep him calm through another long wait Roland tells a story from his childhood. This is "The Wind Through the Keyhole", a fairytale Roland's now-murdered mother recited to him at bedtime, with all the tangled forest, dragons and wicked step-parents such a story requires, and it is what turns this into a gem of a novel, enchanting and enchanted.

Tim is an 11-year-old boy who lives on the edge of the Endless Forest, which is "dark and full of dangers". His father is killed; his new stepfather is wicked; and he ends up on a strange, terrifying, wonderful quest through the forest to save his mother. It begins, says Roland, as all stories do, "Once upon a bye, before your grandfather's grandfather was born". Via a tricksy fairy, a deadly swamp peopled with mutated humans and another starkblast, it ends with an enchanted tyger, magic and Maerlyn.

Wind blows, whistling and magical, through each of Roland's tales, from the ice of the starkblast to the desert wind of the shapeshifter's crumbling home town and the night wind of Tim's woods, which "slip[s] its strange breath over the cottage: sweet with the scent of the blossiewood at the edge of the Endless Forest, and faintly sour  but still pleasant  with the smell of the ironwood trees deeper in, where only brave men dared go". King is utterly at home in Mid-World, and the cadences and rhythms of the vernacular he has created provide a language fitting for the stories and legends he recounts.

King has proved that he does long well. 11.22.63 was a sprawling but unputdownable marathon to save JFK from assassination; Under the Dome a claustrophobic, vintage piece of horror. But when the author reins himself in and keeps it short, he's even better. King writes in his foreword that newcomers will be able to enjoy The Wind Through the Keyhole without reading the other Dark Tower books, but this feels like something of a leap. Better to start at the beginning, or to hold out hope for the book of collected Mid-World fairy stories that Roland mentions, Magic Tales of the Eld.