The Morinus System of Horoscope Interpretation [Astrologia Gallica Book 21] by J.B. Morin, trans. Richard S. Baldwin 
(Washington, D.C.: American Federation of Astrologers, Inc., 1974).

Astrologia Gallica Book Twenty-Two: Directions by J.B. Morin, trans. James H. Holden, 2nd ed. 
(Tempe, AZ: American Federation of Astrologers, Inc., 1994)

Astrologia Gallica Book Twenty Three: Revolutions by J.B. Morin, trans. James H. Holden, 2nd ed. 
(Tempe, AZ: American Federation of Astrologers, Inc., 2003)

Judgment:  Book 21 is absolutely essential for traditional/medieval astrologers, and is recommended for beginners; Book 22 will probably be more of help to everyone due to its many tables of associations; Book 23 is a powerful source of guidance in technique for solar returns, and is more suitable for intermediate and more experienced astrologers. 

If we had to rank the astrologers of history in terms of their known, individual contributions to technique and information, Jean-Baptiste Morin de Villefranche (1583-1656) would undoubtedly be in the top 5, joining Ptolemy, Abu Ma'shar, Bonatti, and William Lilly.  It is a shame that few English-speakers study him, but fortunately individual books of his massive Astrologia Gallica are slowly being translated. 

Long-winded and often preoccupied with defeating real and imagined foes through argument, Morin (Morinus) is admirably clear, concise, thorough and pedagogically helpful when explaining interpretive techniques.  He clearly thought his work would be the end-all, be-all of astrology, and he was ironically correct:  he produced his work at the end of the era of true, traditional astrology (which was all but dead by 1700), and he was able to synthesize the techniques and principles of the astrologers before him in an admirable way.  But Morinus must also be approached with some caution.  Any thinker who sees himself as Morinus did also feels totally free to accept and reject elements of received tradition.  Morinus was a reformer, trying to bring a kind of rationality and symmetry to what he thought were confused areas of astrology, and in the process he cast away certain Greek and medieval elements that he disagreed with.  In this sense he is like Ptolemy, another systematizer.  Morinus rejects terms, decans, the Dorothean triplicity rulers (substituting his own), numerous minor dignities, and pretty much anything he thought was invented by Arabs.  Still, in my view Morinus's work (or what we have of it in English) forms an essential part of the traditional/medieval astrologer's education.

Book 21 (Baldwin translation) is a lovely book that deserves careful rereading.  Some of the material in the first 1/3 of the book will be unfamiliar to most readers because of its preoccupation with contemporary developments in astronomy.  But the book has both a negative and positive argument.  First, Morinus criticizes and often seems to ridicule the reliance on "universal" or "natural" significators in astrology.  Universal significators are planets that are used to signify matters regardless of their placements or rulerships in a figure.  For instance, the Sun is often taken to signify the father.  Morinus agrees that the sun has a certain analogy with fathers, but criticizes the tendency to look to the sun first and foremost when delineating the father -- in this he is criticizing Ptolemy, and his criticism extends to modern psychological astrology as well.  Instead, Morinus explains how to use rulerships and house location to go beyond universal significators.  In the subsequent chapters Morinus shows, with a number of examples, how to judge the following: single planets in a house; multiple planets in a house; interpreting a house when its ruler is somewhere else; how to weigh the combinations of malefics and benefics in good or bad states (dignities and debilities) in the various types of houses; the various types of aspects, especially between the various types of planets.  He is also very helpful when he repeats key phrases in multiple situations or gives multiple examples of the same principle.  By doing this, he helps the student learn the sort of language and phrasing that aids in judging a chart.  He follows up with short sections on synastry, debilities, and other matters.  This book gives the technique and principles that allow a student to read someone like Lilly or Abu 'Ali and understand why they say what they do.

Book 22 is devoted to primary directions, an elite technique with an awe-inspiring reputation.  Most systems of primary directions claim that they have the true version, and especially that they have decoded Ptolemy's controversial passages on them in the Tetrabiblos.  Morinus uses the Regiomontanus system, which is less well-known than the Placidean (but more well-known than the so-called Alchabitius system of medieval times).  Holden helpfully provides the tables and commentary on calculating them.  In Book 22, Morinus often spends most of his time arguing against other astrologers and astrology skeptics, or answering possible objections and questions, although that has a value in itself.  One learns, for instance, that skeptics of old actually studied astrology and knew its jargon; and after reading Book 21, one can instantly see where they go wrong before Morinus unleashes devastating arguments against them.  Holden also includes a number of tables of planetary associations, passages on triplicities, transits, reception, and solar revolutions (among other things) in appendices.

Book 23 is another translation by Holden, this time of Morinus's work on solar and lunar returns (or "revolutions," to use the traditional term).  This is a valuable addition to astrological technique.  Some astrologers of old are content to provide a few examples and general advice, but Morinus provides dozens of delineated solar and lunar revolutions from his own and others' lives.  Contrary to his usual verbose style, the delineations are extremely concise.  This has its downside, because in a revolution one must really delineate two figures: the natal and the revolution -- and then compare them.  There is a lot of delineation and technique behind Morinus's clipped discussions.  Fortunately, Morinus then provides a comprehensive, step-by-step guide to delineating revolutions in the final chapters.  These rules, combined with the rules in Book 21, pretty much give an astrologer all he or she needs to know about delineating revolutions!

All three of these books are valuable, and I recommend them all.  Book 21 is highly recommended for all readers.  Book 22 is valuable for its appendices and the Regiomontanus techniques (difficult though they are).  Book 23 is recommended for students who have already digested Book 21 or know how to delineate a natal figure.