Stephen King is one of the 20th Century and 21st Centuries most prolific writers, so it's not surprising that books exist analyzing his work. Tony Magistrale's book, I understand, is one of many. I've certainly read a lot of King's work, although not all of it, and have enjoyed many more than I found lacking (to be fair, even those books I didn't think were great, were good).
While to a certain degree I enjoyed Stephen King: America's Storyteller, there came a point where I got tired of some of the analysis. At points I found myself wondering if, in fact, sometimes a cigar is just a cigar, and nothing more. While its a fun exercise, unless King pipes in and say, "Yeah, you hit that nail on the head," it's all just one person's evaluation of his work, and may be on the nose or not.
If you're a die-hard fan, check it out, if not, don't bother; turn your attention to King's work itself and discover a writer who has had to fight his way to respectability, based on the genre he has chosen to write in. You're going to really love his stuff. I'd recommend you start with The Shining and move on for there.