The Combination of Stellar Influences by Reinhold Ebertin
For many years this book was never far from my side.  It was written in Germany during the war years by a highly principled researcher-astrologer, Reinhold Ebertin.  I consider it a landmark literary and astrology achievement during our modern times; it is a field-guide to the stars and their effects on our lives.  

There are a lot of books with many pages telling you about what a certain planetary combination means, such as Venus square Saturn in the natal chart, or transiting Sun sextiling Jupiter, or even progressed Uranus squaring the ascendant; and they try to fill your head with expansive, vapory psychological concepts, but they don't tell you what it means in your real life.  Not this book however; it is crystal clear and to the point as to what all possible natal or predictive elements affecting each other mean, both psychologically and materially.