This is an excellent beginning book on breathing for anyone who is interested in yoga. It covers the importance of breathing, basic respiratory physiology and the relationship between science and yoga.
The authors of this book are credible and have excellent professional credentials and established practices. They are associated with the Himalayan Institute in Honesdale, PA that has a long standing commitment to getting science, modern psychology and yoga together. Many of their publications are excellent and certainly this one is as well.
I think any yoga practioner at any level will benefit from this book and even very experienced people will learn a lot. As the title suggests, this book focuses on the practical aspects of breathing and provides a good foundation or launching point for moving on to pranayama practice. However, the value add is the tie in to modern science.
As part of my living, I do workshops on such topics as meditation, pranayama and biofeedback, loving-kindness practices and other topics that integrate East and West. Before becoming involved in psychology and spirituality, I was both a research scientist and Teaching Fellow in Physiology. While I am not an expert at respiratory physiology or a top pranayama practioner, I have an established yoga practice and enough expertise to judge the merit of this book.
If you are serious about the breath and especially about pranayama, I would pick up "Light on Pranayama" by B.K.S. Iyengar. This book is very comprehensive and covers the physiology of the various breathing practices in great depth. If you are looking at breating for relaxation, you may also want to consider some of the meditation resources listed on my listmania lists. I also have a yoga resource list that might be helpful for those interested in pranayama. Another book in the pranayama vein that is quite popular is "The Yoga of Breath" by Richard Rosen.
In summary, this is a good book on the basics of breathing as a foundation for a yoga practice. It provides the basic concepts required to move on to more advanced breathing techniques such as pranayama.