The Human Aura: Astral Colors and Thought Forms by William Walker Atkinson

In this book, I speak of the human aura, and its colors, as being perceived by astral or clairvoyant vision, for this is the way in which it is perceived and studied by the occultist. The occult teaching is that, in the evolution of the race, this astral vision will eventually become the common property of every human being — it so exists even now, and needs only development to perfect it.
William Walker Atkinson (December 5, 1862 – November 22, 1932) was an attorney, merchant, publisher, and author, as well as an occultist and an American pioneer of the New Thought movement. He is the author of the pseudonymous works attributed to Theron Q. Dumont and Yogi Ramacharaka.