Not just a book on yoga. It probably deals more with meditation than yoga, with a cosmic dash of energetic feels thrown about. Gives a bit of a different perspective on yoga than modern ones. Considering this book was first published in 1971, this shouldn't be too much of a surprise. Religion and spirituality has to change a lot in four decades to keep up with modern times.
Overall, it approached yoga from the Indian philosophy and religious viewpoints. It's all about union with the universal soul. But, on the physical level, postures aim at physical well-being and control over the body. A healthy body is a prerequisite for a healthy and controlled mind. You don't hear too many new age folk talking about control or perfection these days. But the teaching was there in the 70's. All forms of yoga "lead to the state of perfection, wisdom, and bliss."
Oh, and can't forget the string and cloth cleansing recommendations. Gajakarani and dhauti, respectively. Crazy fuckers in the 70's, man.