This is a “novella” and I don't exactly know how long it is, because I read it on the iPhone's Kindle app. It took about as long to read as a short book so I am going to count it. The concept is that an English teacher buys a Kindle that allows him to purchase books from alternate realities.
King is at his best when he produces novellas and short books. He can be a sloppy writer but is better than common wisdom would have it, and can still come up with killer sentences like this one:
“They both wore long mustard-colored coats, the kind that are called dusters, and Wesley understood, without knowing how he understood, that the coats were alive.”