I'll review "On Writing" in two parts, since the it's essentially a hybrid between a memoir and a manual on writing. I'm starting with the memoir sections.
What I love about "On Writing" time and time again is how it doesn't read like a textbook. It's not extremely focused on the craft as other how-to books on the market and while it's a drawback for writers who want in-depth advice and a tutorial, you should really pick it.
More than the half the book pieces together King's past, his childhood, his adolescence, his college years and the years before becoming... well, King.
There is no direct lesson. "On Writing" easily reads like a memoir and for all King fans I imagine it will be a treat. However, apart from learning more about King, there are subtle points that he makes. I group and label those into an 'emotional workshop'. For starters, sharing all his personal tidbits and how his ideas form underlines how different all writers are and while the market is influenced by trends there is no master mold.
Your big break maybe won't be through a novel, but through short stories. Or vice versa. I can even say that you could very will be published with your experiment in a different genre, for a different audience or a different medium [King has written for newspapers as a young one]. Your introduction to the publishing industry depends on chance. It may be months after you've decided to pursue this path or it may take years.
King's also saying that every writer will differ from every other writer on the planet. Every writer is a conglomeration of experiences and it's guaranteed that every writer's voice will be strictly individual. What else comes with this individuality?
The confirmation that universal advice on writing is non-existent.
But perhaps the most imporant advice here is to never give up, when pursuing a career in writing [or in any art for that matter]. This is mentioned in the first one hundred pages, in which King describes in detail how he worked in a laundry shop before his big break. Up until that point, Stephen King was not much unlike any of us, doing something we don't like so that we don't starve and support a family. You don't know whether your dream is a realistic one. No matter how bleak the situation may feel, the lesson is to keep writing.
Some writers give up after bitter disappointment, but once you've started and you KNOW there is nothing other than writing, it's better to keep going. Because you never know.
In the postscript, King relays how he was run over, his hospitalization, his fear that he may not live long enough to see his wife, how writing managed to bring him back to life. I think this is not much what you have to do for the sake of writing, but what writing has to do for you. Writing has to be fun. Writing has to uplift you. Writing has to keep you sane, while you drag other people through hell. Writing has to save you.
If writing does all that for you, then it's worth all the sacrifices you are willing to make for it. The forceful late night writing sessions, when you feel like crap. The nerves of getting it wrong. The pain of rejection. All worth it.