I remember hearing about the story -- and the by-all-accounts-godawful film version of it -- and wondering How can there be enough story in that for a novel? Two people trapped in a car by a rabid Saint Bernard? That's it? Of course not, and touching on more of the characters of Castle Rock and their various dramas is part of what makes the book work enough. It would be hard to have a short story of the same situation have the impact that the novel's ending has, even if the short story had the same ending. The novel, all 304 pages of it, had earned that reputation of being unsparingly brutal. Here's one of my previous encounters with that reputation: someone I know tried reading Cujo and could not finish it. Now this friend is not usually a horror reader, but when she first read docbrite, she chose (because it was the only one of Brite's books at her library at the time) Exquisite Corpse. And was very impressed with it. Couldn't handle Cujo. And now I better understand why.
Here's part of what works about Cujo: the book feels grimy. It's set at a bad time in the "history" of Castle Rock, Maine, when the town feels a touch uglier than usual. It focuses on the trash floating on the edge of lakes. The few purely lovely images are thanks to dream imagery. And then when you're in the head of the rabid dog Cujo (did people call their dogs Cujo before this book? Did anyone call their dogs Cujo afterwards?), it manages to get uglier.
Stephen King can be especially good writing about ugliness. Also unfairness: the tone of much of the last fifth of the book is How could this have happened? Why? Circumstances fall into place in such a way, a certain way, and two characters are literally trapped by what could seem like contrivance in lesser hands, and other characters are metaphorically trapped by the contrivances surrounding them: they're not threatened with death by rabid 200-pound dog, but they're threatened by other ugliness. This is a book King wrote during a profoundly (it's worth calling it) fucked-up time in his life; as he admitted in 2000's On Writing, "I barely remember writing [it] at all. I don't say that with pride or shame, only with a vague sense of sorrow and loss. I like that book. I wish I could remember enjoying the good parts as I put them down on the page." He could still do what he does even when he was too drug-addled to, maybe, know fully what he was doing.