Stephen King is entertaining in the entire novel, even as he explains his do's and don't's list of writing mechanics. This leads me to believe that the tone is mainly light-hearted. Some examples of this are found practically everywhere throughout the book;
On page 141, Stephen starts out his spiel about writing by using an analogy about how unlike the fact that there are no bad dogs, there are bad writers. He does this though in a joking manner, to almost cushion the blow;
There are no bad dogs, according to the title of a popular training manual, but don't tell that to the parent of a child mauled by a pit bull or a rottweiler; he or she is apt to burst your beak for you. And no matter how much I want to encourage the man or woman trying for the first time to write seriously, I can't lie and say there are no bad writers. Sorry, but there are lots of bad writers.On page 220, King tells about how his wife Tabby was reading one of his manuscripts in the car. King goes on to say that he had kept peeking over to her to see if she had at least smiled at the humorous parts of the story. His wife caught on after about the 7th or 8th time (King reluctantly admits that it may have been his 15th) and yelled at him "Pay attention to your driving before you crack us up, will you? Stop being so goddamn needy!" King concludes this memory with the the confession that most writers are needy, especially between the first and second drafts.
The last example can be found on page 35, when King reminisces in a childhood memory of his favorite television shows. He explains the first story that he ever sent in to a magazine for a contest, and also the kind of child he was when he wrote the story. "I don't recall the title"- by this he means the title of the story he sent in- "but I was still in my Ro-Man phase of my development, and this particular tale undoubtedly owed a great deal to the killer ape with a goldfish bowl for a head".