From King’s website:
The major story of Roland and his ka-tet was told, but I realized there was at least one hole in the narrative progression: what happened to Roland, Jake, Eddie, Susannah, and Oy between the time they leave the Emerald City (the end of Wizard and Glass) and the time we pick them up again, on the outskirts of Calla Bryn Sturgis (the beginning of Wolves of the Calla)?
There was a storm, I decided. One of sudden and vicious intensity. The kind to which billy-bumblers like Oy are particularly susceptible. Little by little, a story began to take shape. I saw a line of riders, one of them Roland’s old mate, Jamie DeCurry, emerging from clouds of alkali dust thrown by a high wind. I saw a severed head on a fencepost. I saw a swamp full of dangers and terrors. I saw just enough to want to see the rest. Long story short, I went back to visit an-tet with my friends for awhile. The result is a novel called The Wind Through the Keyhole. It’s finished, and I expect it will be published next year.
It won’t tell you much that’s new about Roland and his friends, but there’s a lot none of us knew about Mid-World, both past and present. The novel is shorter than DT 2-7, but quite a bit longer than the first volume—call this one DT-4.5. It’s not going to change anybody’s life, but God, I had fun.
Ahh, I’d almost forgotten about this. I’m of the mind that King’s direct tales relating to The Dark Tower were almost perfectly concluded with the final volume and are in no need of expansion. Despite King claiming holes in Roland’s story, sometimes the mystery of untold adventures is better than actually hearing them told.
It’s strange to me that King is going back to Roland and his ka-tet; though, for someone who’s been telling that tale and living in that world for nearly 30 years, I suppose it’s hard to leave fully behind. The aforementioned hole was left in the story at the time of writing, so I don’t see why, all of a sudden, it’s important enough to tell, especially if it ‘won’t tell [us] much that’s new about Roland and his friends’. The writing of this series has been anything but steady and consistent, however, so I’m not surprised to see King jumping around in the telling of it. If King had fun writing it, hopefully we’ll have fun reading it. I’ll be approaching it with cautious optimism.