I first read about Horary Astrology in the mid-1980s. I was learning all that I could about astrology, but could find nothing to really TEACH me how to DO horary.
Then Anthony Louis' first book on Horary, this complete textbook (and I DO mean complete) was published, and the mystification of this subject was dispelled. In addition, I also gained an extensive background in traditional astrology, which has served me well not only in horary, but in medieval and hellenistic astrology as well.
If you buy only one book on horary, make it this one (the 1991 edition with the brownish-orange cover, consisting of almost 560 pages).
He has since published other books on horary that are no less clarifying and helpful; but since this was my introduction to the art and practice, and it is packed with information, this one is my favorite.
After studying this book I became proficient in this art. I have answered dozens and dozens of questions, and have found many lost items, using horary. I thank Anthony Louis for being the first and main source of my training in it.