(This post is practically spoiler free)
I have started the new book - The Wind Through The Keyhole.
It fits in after they come out of the green castle near the first thinny and before they get to Callah.
It's pretty good, so far. I had to stop and think about a few of the mid-world words, but then it clicked and they all made sense. It starts off with the ka-tet, but then they have to stop and Roland starts telling them a story, which I think is going to be the meat of the book.
In theory, King can keep writing them forever. Good, say I.