This book is reprint of the Dover Edition from 1959 which was itself a reprint of the book first published in 1937. The book is also called "Yoga: Its Scientific Basis."
The evaluation (circa 1935) seems quaintly unscientific today. Some of the chapters are "Yoga and Psychoanalysis" and "Yoga and Psychic Research." Dr. Behanan, an Indian, took up the writing and study as Sterling Fellow in the Psychology Department at Yale. The presentation is from that viewpoint. The "scientific" evaluation is more or less tacked on. He gives a favorable opinion of yoga, although he doesn't seem to think that the practice is more than a psychological phenomenon. Of course the philosophy is very real.
Probably the best chapter is the first one: "The Apparent Complexity of Indian Culture" which gives a brief history of not only Indian culture but Indian religious philosophy from pre-Vedic times through the Upanishads and the Buddhist "heresy" which was a reaction to the overly ritualistic Brahmanism. It should be realized that yoga is not only a methodology toward mental and physical health, and a means to liberation from our bondage in this world, but it is also one of the six orthodox philosophies of India.
Behanan also presents Patanjali's program from his famous sutras. There are illustrations of some of the postures and an introduction to pranayama is given. Behanan explores some of the central ideas of Indian thought such as Brahman and Atman, karma and reincarnation. Chapter Two is devoted to "prakriti" (nature, the insentient, or the phenomenal world) which is contrasted with "purusha" (outside of nature, the transcendental Self) which is the subject of the third chapter.
The book minus the two extraneous chapters mentioned above serves as a reasonably good introduction to yogic philosophy. The tone is academic but the ideas are easily accessed by a general readership.