Having been a traditional astrologer for the past 15 years, I've build up quite a library! And while you can get by with just a few key books, it really helps to have all your bases covered, there's so much out there.
We are really blessed to be living at a time when so many traditional astrological and magical sources have been translated into English.
I've decided to start a regular traditional astrology and astrological magic book recommendation and my first suggestion is Al-Biruni's Book of Instructions in the Elements of the Art of Astrology
The astrology section is available on Amazon for just $10 and change
It's also part of my Renaissance Astrology CD Library III which has the complete book, though most of it I've never had occasion to use because it is either mathematical, astronomical or how to use an astrolabe!
You can read about Al-Biruni on my web site here
The Book of Instructions is really a key text for any traditional astrologer. Al-Biruni lays out a very early Arabic version of the basics of traditional astrology, signs, essential and accidental dignities, rulerships, etc. He gives very clear and very straightforward explanations. I've chosen Al-Biruni as one of my three main sources for planetary rulerships, along with Agrippa and Lilly. A great reference and very useful introduction to Arabic astrology, the root of traditional European traditional astrology.
Definitely worthwhile to get a copy of the Book of Instructions!