A healthy lifestyle means breathing well - but most people breathe badly. To breathe well only means not to obstruct the natural processes, including breathing through the nose, and to develop a posture that allows this, because naturally we know exactly how to breathe. We just forget, and inhibit ourselves.
The yoga principle of pranayama is explored in detail here, and yoga postures that assist in good breathing make this an excellent 'remedial' book - but primarily it is a book which simply allows, and seeks, a return to nature.
In our post-pandemic world understanding the breath has a whole new relevance. The book also covers anatomy, chanting, and meditation or mindfulness.