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To give you a quick answer, I am grabbing a quote from Maurice McCann, published in his ?Void of Course Moon?:

"?it [Christian Astrology] is the most complete and influential book on horary today. There is no other book that does not owe a debt of gratitude to it, or can rival it. Besides, there are no other known sources where so many horoscopes can be studied for their use of rules and resolutions to questions. This book is widely used throughout the western astrological world and has a major influence on astrologers. It is generally believed that various technical problems can be solved through a study of Christian Astrology. However, it requires skilful reading between the lines to understand Lilly as he was never a lucid writer."

Lilly was the first traditional astrologer to write a comprehensive textbook in English rather than Latin, so his work became the most accessible for subsequent students. He translated the works of other horary authorities such as Al Kindi, Masha?allah, Bonatti, many of which are now becoming available through translations, so we now have other important horary authorities to refer to besides Lilly. But as McCann points out, whereas these other texts clarify the theoretical principles, they fall short on practical demonstrations. Lilly included dozens of examples from his own files. He was also a very prolific writer and an extremely experienced astrologer. It has been reported that in one year he worked an average of 5 charts a day. If you want to know more about his life and work see http://www.skyscript.co.uk/lilly.html