The most important tool for an astrologer is the emphemeris, a book which lists planetary positions and phenomena.  On the cusp of the millenium you will need two books:

American Ephemeris for the 20th Century : 1900 to 2000 at Midnight by Neil F. Michelsen.   You need this one to look up the planets, eclipses, and retrogrades for everyone who has been born so far in the twentieth century. (This ephemeris is also available for noon.)

Even if you have a good calculation program, you need an ephemeris.  If you have the top of the line calculation program, it probably came with or you purchased the U.S. and World Atlas.  If not, you need these atlases, not only to look up the longitude and latitudes for correct calculations but also the time zones in effect at the birth date.

The American Atlas : U.S. Longitudes & Latitudes Time Changes and Time Zones
                     by Thomas G. Shanks (Compiler)

The International Atlas : World Longitudes & Latitudes Time Changes and Time Zones
                     by Thomas G. Shanks (Compiler)

If you're the old-fashioned type or planning on taking a professional astrology certification exam which includes calculating the horoscope by hand (hey -- we did it for years before the home computer was available), you will also need a Table of Houses:

The Michelsen Book of Tables : Koch and Placidus Tables of Houses
How to Cast a Natal Horoscope; Interpolation Tables; Time Tables
                     by Neil F. Michelsen (Compiler), Robert Hand, Josh Brackett

If you want to understand the horoscope you've just calculated and constructed read
Horoscope Symbols by Robert Hand.  Not a book for beginners, it's a book for the astrology student who is beginning to go in depth.  You will find explanations of retrogrades and house systems, for example, along with core meanings for planets, signs and houses.

Rulership Book : A Directory of Astrological Correspondences by Rex E. Bills, The Book of Rulerships : Keywords from Classical Astrology by J. Lee Lehman, and Astrological Keywords by Manly Palmer Hall are helpful resources to figure out what planet or sign rules what factor in our everyday world.