We are planning to put together a brand new encyclopedia of astrology. A good starting point, for example, would the Astrology entry in the Gale Encyclopedia of the Unusual and Unexplained (by A great resource is Nicholas deVore's Encyclopedia of Astrology (by ZET Astrology Software), also available as the Astrological Dictionary (by Astrology Weekly). Yet another online source is Astrowiki (by Astrodienst). By way of a counter-example, the Astrology article by Wikipedia has a strong anti-astrology bias.

The Tetrabiblos is attached below as a reference (courtesy of Classical Astrologer)