The essays appeared in The Theosophist, the Society's journal at Adyar, between 1887 and 1889, and were issued as a book in 1890 under the title Nature's Finer Forces: The Science of Breath and the Philosophy of the Tattvas. The argument is that the five tattvas, akasha, vayu, tejas, apas and prithivi, are not abstract categories but working modes of a subtle substance; that each is dominant in the breath for a fixed period in turn; and that knowing which is active tells you what an action begun now will come to.
The material is not his invention. He was drawing on the Sanskrit literature of svara, the breath, and on the prognostication built on it, a body of practical texts with a long history of its own in India. What he added was the presentation: a systematic exposition in English, with tables, aimed at readers who had heard the word tattva and had no way into the thing.
Reception inside the Theosophical Society was not uniform and the book was argued over there as well as outside it. What is not in dispute is that it was read, and that it was read by exactly the people then assembling a Western magical system out of whatever Eastern material came to hand. That timing, more than the content, is why the book matters.