The natal chart can be used to read the past-life incarnations as well as the tendencies for future incarnations.  What we are today is a result of our past thoughts, actions, and desires, either during this lifetime or in previous incarnations.  We base our decisions and responses to life on these past influences. This is Karma, a fundamental law of the universe.  Understanding the past gives us the ability to use wisdom in our life choices and facilitates our positive transformation.

No one book has all the answers regarding karmic/past-life astrology.  There are many different techniques and ways of looking at the horoscope to determine the forces of karma and the lessons for this incarnation.

Martin Schulman has written a series of books on this topic, including Karmic Astrology : The Moon's Nodes and Reincarnation and Karmic Astrology : Retrogrades and Reincarnation.  It's pretty simple to look at your chart and find the nodes or your retrograde planets and just read the sections in the books that correspond.

2007 update:  Martin Schulman's books are available as downloadable audiobooks at Classic Legend Books.

Astrology and Reincarnation by Donald Yott is another book that stresses retrogrades and also talks about the karma and interceptions.  More on the moon's nodes and patterns of reincarnation are found in Astrology for the Soul by Jan Spiller.

Other recommended books on Karma, Reincarnation & Astrology are:

Astrology and Past Lives by Mary Devlin

Evolutionary Astrology : The Journey of the Soul Through States of Consciousness
by Raymond A. Merriman

The Karmic Journey; The Birthchart, Karma, and Reincarnation by Judy Hall

Manly Palmer Hall's Astrology and Reincarnation may not be out of print but I can't find it at Amazon or Barnes & Noble.

Out of print:  Astrology and Your Past Lives by Jeanne Avery