Let's be honest: Stephen King is not one of the greatest writers of all time. He will never win a Pulitzer or a Nobel (he might win a Newberry though, if he ever decides to tap into the Kids/Young Adult market), and on the few times his books are featured in the New York Times Book Review, the reviewer will treat the book with a sort of haughty disdain, knowing their time could be better spent trashing Joyce Carol Oates.
None of this should suggest, however, that King is not qualified to write a book about how to write. Sure, he churns out pulpy horror stories that are proudly displayed in airport bookstores, but the man knows how to write a good story, and he's probably one of the most well-known, non-dead American authors in the world. So he must be doing something right.
I'm not the biggest fan of King's books, but I really enjoyed On Writing. He talks about writing frankly and practically, mixing tried-and-true pieces of advice (fear the adverb, never write "replied/remarked/muttered/yelled etc" when you can write "said", and don't be afraid to kill off your favorite character) with anecdotes about how some of his books came about. I especially liked the story behind Carrie: King was working as a janitor at a high school, and one night he was cleaning the girls' locker room. He asked the other janitor what that little metal dispenser box on the wall was, and the other man replied that it was for "pussy pluggers." At the same time, King had been reading about how psychic abilities often manifest in girls just beginning to go through puberty. He combined the two ideas and wrote out a couple pages that would turn into the opening of Carrie. (if you haven't read it you should.) Many thanks to King's wife, who rescued the pages from the wastebasket after King first decided that the idea was stupid and threw them away.
So, in conclusion: even if you aren't a fan of Stephen King's work, he has some very good advice about writing and storytelling, plus some good stories of his own. Sure, you can call him a sellout. But I like him.
Also, he once said in an interview that Stephenie Meyer "can't write worth a darn." You stay classy, Mr. King.