King book that features an author writing a book, in his head, on a long drive?

I feel like I'm losing my marbles - I thought this was Bag of Bones, but I'm flicking through it and can't find it. There's a book by the Big Man that features an author (big surprise) and he's driving somewhere, thinking up a story. IIRC it's a crime novel, or a detective story, and this author doesn't usually write those kinds? It might even show up in the book with that typewritten font SK likes to use on occasion.

I don't think it's Misery, because he's already finished writing when the book starts. The Dark Half has an author but again I thought all that happened before the thrust of the book begins.

Any ideas?