A good book on just signs is All Around the Zodiac by Bil (with one L) Tierney.
When I was just starting out with tarot, I got alot of use out of Keywords for Astrology by Hajo Banzhaf and Anna Haebler.
And, if you're a cheapskate like me, these are on discount at discountnewagebooks.com:
Llewellyn's New A to Z Horoscope Maker & Interpreter, Retail Price $19.95, Your Cost $5.99. A good compendium with a little of everything, including all the planets in all the signs.
The General Principles of Astrology, Retail Price $80.00, Your Cost $49.99. Ghost-written by Uncle Al himself!