Christian Astrology, published in three volumes in 1647, was the first complete astrological textbook written in English rather than Latin, and it made the technical apparatus of the art, previously restricted to those with a university education, available to any literate reader. Its middle volume, devoted to horary astrology, the practice of answering a specific question by casting a chart for the exact moment it is posed, gave the technique its most systematic treatment in the language and remains, more than three and a half centuries later, the standard reference horary astrologers train from.
Lilly wrote in a direct, example-driven style, working through real questions he had been asked, lost objects, absent lovers, sick clients, side by side with the rules for judging them, which is part of why the book has stayed usable where more theoretical treatises of the period have not. The twentieth century revival of horary astrology, led by astrologers such as Olivia Barclay, worked explicitly from Lilly's text, reprinting Christian Astrology and building modern teaching curricula directly on his method rather than on any intervening tradition, so that a technique three hundred years out of print became, again, a living practice taught largely in Lilly's own words.