I would suggest Shannon's book, Analogies, Energies and Celestial Bodies which she excerpts in this article. It is my new favourite astrology book.

The book that originally taught me about astrology and that I still consult 30 years later is called The Only Way to Learn about Astrology by Marion D. March and Joan McEvers. Unfortunately it is out of print, but it is available second hand online. It brilliantly explains how each planet affects whatever sign or house it is in.