Skip the autobiographical bits - which cuts the book in half - but the much slimmer book that results is the only book about writing worth reading besides Strunk & White's Elements Of Style. It's truly excellent, and one of the best books I've read in my life.
I tracked this down in Powell's following RailsConf, partly for the airplane read and partly because of a magnificent essay King wrote on imagery, which I found on the Web last year.
Put your vocabulary on the top shelf of your toolbox, and don't make any conscious effort to improve it...One of the really bad things you can do to your writing is to dress up the vocabulary, looking for long words because you're maybe a little bit ashamed of your short ones. This is like dressing up a household pet in evening clothes. The pet is embarrassed and the person who committed this act of premeditated cuteness should be even more embarrassed.
I've always ignored King, both in a snobbish way and because three pages of The Dark Half gave me a truly epic nightmare when I was in high school, but I went on from this to Danse Macabre, a soliloquy on horror fiction in its various forms, and it's amazing as well. The section on the history of horror movies has some of the best writing about film I've ever seen. (Although to be fair I still haven't read anything by the great Pauline Kael.)
Notable in Danse Macabre: Alien is Lovecraft in space, the abandoned spaceship a long-dead temple of the Elder Gods; Job from the Old Testament is "the human Astroturf in a kind of spiritual Superbowl between God and Satan"; and Andy Warhol's soup-can paintings are actually single-frame horror movies with a dollar-shaped monster.
I also picked up Firestarter but haven't started into it yet. I went with that as my first attempt at a King novel in nearly twenty years because it's so obviously the prototype for Firefly's River Tam.
Anyway, blah blah blah Stephen King, but I have to say, if anybody's concerned with being a good writer, it's Strunk & White Elements Of Style and Stephen King On Writing. (Aristotle's Poetics too, of course, but hopefully it goes without saying that people should read Aristotle.)