One of the best astrology books ever written -- discusses the aspects individually, along with all potential midpoint combinations. This tries to empirically link astrology, geometry, and archetypes.

This is undoubtedly one of the best modern books written about the aspects; it offers solid interpretations about each and every aspect combination, though more in an essay format.

A decent text/cookbook -- it is incredibly staid, but luckily it includes the quincunx as a major aspect and attempts to offer us the meaning behind that elusive 150 degree aspect.

This focuses more on aspect theory instead of individual interpretations -- I cannot praise this text enough though. This is the PERFECT BOOK to start with if you are interested in SERIOUS astrology.

This book discusses the astrological aspects rather profoundly, though not exclusively; it covers all major facets of astrology as well -- an excellent reference book and recommended for the beginner.

This text owes a lot to Johannes Kepler, who was the forerunner of 'Harmonic Theory' and argued for a stripped-down astrology devoid of the houses and even the 12 astrological signs. I'm all for it!

Written by one of the foremost astrologers in the modern world, this book examines the effect of transiting (orbiting) planets as they 'trigger' a person's natal planets.

A wonderful (though expensive) all-around guide to astrology -- better yet, it includes an extensive section dealing with the aspects and midpoints.

...more focus on aspect-patterns (when you are struggling to understand that wacky and extremely rare Grand Quintile found in your astrological chart!).

In this book many of the aspect interpretations are a bit on the fatalistic/'Karmic' side -- unfortunately, most of it seems to have been ripped off from Grant Lewi's "Astrology for the Millions."

This is a Dane Rudhyar classic that is focused on theory and harmonics; he focuses on the technical/geometrical rather than re-writing endless amounts of canned interpretations for the masses.

This is the same author and series as the above book, yet this time he focuses his laser-like attention on the inconjunct (AKA the quincunx), an aspect neglected for most of astrology's history.