It's very difficult to imagine Judith Viorst writing a bad book. She is an excellent writer and gears her writing toward the existential crisis' of life, development, etc. As some reviewers suggest, she backs up what she say with a fair amount of references to psychological literature, but she is certainly credible and although the book doesn't read like a dime store novel, it's also very accessible to the average adult without any psychological training.
The dance between control and surrender is very difficult for most people and comes up in couples, the workplace and other areas of life. This is the main topic of the book and describes why this is so and how to work with it toward achieving peace of mind. Much of this work has to do with reframing the human condition and daily struggles we all face in a more realistic fashion. It's about finding the balance point between control and surrender moment-to-moment.
Some of the areas of particular interest to me were the chapters on taking possession of ourselves, who controls the couple, the power of sex, permanent parenthood and in control of death. This is all practical and serious stuff and often areas where people confront considerable pain. While this book or any book doesn't provide all of the answers on these difficult life questions in terms of surrender and control, it will give you powerful insights and more flexibility for dealing with them.