Like most good literature, the reader experiences layers; on top, characters involved in a plot, but, down deeper, characters learn and change through their experiences.
Here, it's no spoiler to simply say it's a book about a guy who's mysteriously losing weight, no matter how much he eats. But, down below, it's about sometimes learning to change and sometimes accepting things that can't be changed.
Stephen King is still known as a "horror" by people who haven't read his later works. His later decades have shown his capabilities to be much wider. Not a drop of horror in this book, and no room for it; it's too full of warmth, growth, acceptance, and, most of all, the importance of friendship.