Bev Vincent sent us some additional information discussing the differences between one of his previous books, The Road to the Dark Tower, and his new one, The Dark Tower Companion: A Guide to Stephen King’s Epic Fantasy for those who are curious:
“When I wrote The Road to the Dark Tower, I started off with the assumption that the reader had already finished the series. I was quite clear up front: venture 
beyond Chapter 1 and you will meet extensive spoilers about the series finale. The only way to discuss the implications of certain things was to reveal details from later in the series. The Road to the Dark Tower is analytical, delving into the meaning of things. I think of myself as a tour guide, pointing out things readers might have missed or not appreciated on the first pass.
The impetus for The Dark Tower Companion was the then-announced movie adaptation, which was supposed to be out this year. I thought that there would be an audience of people who would be introduced to The Dark Tower without ever having read the book. The Companion would be a handy reference guide for them. It quickly orients people to what the series is all about.
However, as I worked on the book, it became more than that–it covers just about everything related to the series, but is much less analytic than The Road to the Dark Tower. You can safely read much of it without being “spoiled.” If you can’t remember who a certain character is or in what book(s) they appeared, you can easily find that information. It’s also the first book to address the Marvel adaptations and their place in the mythos. It’s full of interviews, too–including one with King. I also interviewed Ron Howard and Akiva Goldsman about their plans for the movie and they were very revealing.”
