The Wind Through the Keyhole by Stephen King
Chronologically the book takes place between books 4 & 5. Roland and his tet are taking shelter along the path of the beam from a deadly storm called a Starkblast. While they're there Roland relates two stories, or rather, a story that includes within it, the telling of another story. The story he tells took place when Roland first became a Gunslinger. He and Jamie DeCurry were sent to Debaria to look into reports of a skin-man (a shape-shifter), who had killed dozens of its residents. A young boy named Bill Streeter survived one of the attacks and Roland needs him in order to identify who the skin-man is while in human form.
While he's keeping watch over Bill one night, he tells him a story that comes from the Magic Book of Eld. That story is about a young boy who went on a dangerous journey in order to seek out the magician Maerlyn who he believes can cure his mother's blindness.
Each of the stories contained within this book is great. The book illustrates once again why King is head and shoulders above all his peers. If you've never read the DT series, I HIGHLY recommend it. But not until you've read other books by King that tie into the series first: The Stand, Salem's Lot, Insomnia, It, The Eyes of the Dragon, & The Shining. There are several other books and short stories that have a connection to the series, but the connections are pretty superficial and it really doesn't matter which you read first.
★ ★ ★ ★ ☆