The study of dreams has often been the province of Freudians and flakes. And it's true that it could use a dose of Head and Shoulders. But there is substance there, too.
Dreaming seems to be important for health. Test subjects who get adequate sleep but are denied the opportunity to dream do more poorly in performing tasks, memorization, etc. It may also be helpful in problem-solving. When I haven't been able to figure something out, I've sometimes been advised to "sleep on it." Turns out, that's good advice. (For me, it would have been helpful to add, "but not during class.") Creators such as writers, artists, and composers often credit dreams for inspiration or for specific elements of their work. There is also evidence that the dreaming brain does sometimes produce solutions to more mundane challenges.
Most dreams are not nightmares, but usually, they aren't pleasant.* The book suggests that this may be the brain's way of preparing us for the worst. In a risk-free environment, we can experience something we fear, become more comfortable with it, maybe even develop some coping mechanism.
Freud tried to shoehorn dreams into his psychological theories, but he was right that there is potential for better self-understanding through dream interpretation. Besides therapy, the book explores dream groups, who meet more or less formally to try to understand the members' dreams. As a social mechanism, dream groups sound like they are very effective. I'm not sold on the interpretations themselves.
The book concludes with an exploration of lucid dreaming, where the person is aware of being asleep and dreaming. In some cases, the person can steer the dream. I have experienced lucid dreaming, and it is very cool.
A lot of the book is back-to-back descriptions of experiments and results. Many of these are problematical to me, because (1) they're almost all self-reported, and (2) it's difficult to tease out cause and effect--did I become calmer because my nightmares stopped, or did my nightmares stop because I became calmer? But I did find myself convinced that if I wanted to, I could improve my recollection of dreams and increase the probability of lucid dreaming.
*My recurring dreams are about being in school and not knowing where my locker is, what the combination is, or where and when my classes are. I learned from this book that this is so common as to be a cliche'. Hey, I don't read books to be insulted.