The books referred to by Moongold are excellent, I have the Arroyo book and have dipped into the Schulman series.
However I don't think that natal charts demonstrate Karma in the sense of an open book. As Astraea and Dave say, there is a need to use intuition and an in-depth discussion with the querent, though certain themes may well appear in the chart - they need to be explored and reflected on. Even so by it's very nature, past life information cannot be verified. So the value of past life information does not lie in the empirical field, it lies in the intuitive, psychic, soul centred nature of our lives - so it can be extremely valuable to the way in which we conduct our lives.
I'd also echo Dave's point about perhaps trying past life regression or finding a good reptuable medium who does psychic development classes. The Astrological chart then becomes something of a stimulus to psychic connections, rather than the hard evidence of what we did in earlier lives.