for Dark Tower fans, anyway.
It was a story inside a story inside a story.
The outer wrapper was Roland and his ka-tet just after the end of _Wizard and Glass_, and contains spoilers for that book. That wrapper is pretty thin, though--they ease on down the road and get caught in a violent winter storm (called, appropriately enough for Song of Ice and Fire fans a "starkblast" and while spending the night in an abandoned building next to a fire, Roland tells a story. (About 1/8 of the book.)
The story he tells is of his youth when, again just after the flashback events of _Wizard and Glass_, he is sent by his father with Jamie DeCurry to a neighboring barony to track down reports of a "skin person"--potential shape shifter. They do as young gunslingers should in restoring order in a world moving on, and Roland does learn more about his mother. (This is about 3/8 of the book.)
This story is split in the middle when young Roland tells a frightened kid a story that his mother read to him often, "The Wind through the Keyhole". This is placed back in "once upon a bye" and is a story of a youngster, surprise, dealing with his fears and a bad situation and taking it on. It is a full half of the book and the touch and tone is very much like King's other fairy-tale type books: paced, direct, matter-of-fact, and marvel-filled. (If you've read them, you know what I mean. If not, trust me, he's not all about suspense.)
Overall it adds nicely to the Dark Tower canon. However, someone just reading the basic seven books isn't going to miss this one. King says in his introduction that it was pleasant discovery for him that these characters hadn't finished speaking yet.
And I'm always up to sit up late at a fire and hear Roland spin a yarn.