Answer:
   
   It's important to differentiate between the popular notion of Kabbalah
   and the concept within traditional Judaism. In the popular culture,
   Kabbalah is perceived as a form of magic or the occult, studied for
   selfish personal gain. This misinformed idea resulted from those who
   adapted Jewish ideas out of the context of Jewish belief and practice,
   warping it away from its foundations to their own purposes. These
   include medieval Christian mystics, neo-pagan groups, and contemporary
   "new age" movements.
   
   Within Judaism, though, Kabbalah is the part of Torah that addresses
   the process of creation ("Ma'aseh B'raisheet") and the relationship
   that G-d maintains with creation ("Ma'aseh Merkavah"). As such it is
   the Torah's inner aspect. Some traditions say that some of the key
   texts go as far back as the Patriarch Abraham.
   
   Parts of Kabbalah, such as the [5]Zohar and Rabbi Moshe Cordovero's
   "Pardes Rimonim," are accessible, but difficult to understand without
   a firm grounding in the more basic Jewish sources and an informed
   teacher. Other parts remain hidden and unavailable to the public.
   Parts have been committed to print, but others remain as closely held,
   orally transmitted tradition.
   
   The most accessible, traditionally accurate books for English language
   study of the topic are Rabbi Aryeh Kaplan's "Innerspace, Introduction
   to Kabbalah, Meditation and Prophecy" (Moznaim Publishing, Brooklyn
   NY), "Meditation and Kabbalah," "Kabbalah and the Bible" (Samuel
   Weiser and Sons, New York), and "Jewish Meditation" (Schocken, New
   York). Lubavitcher Chassidim recommend directed study of the Tanya.
   (Kehot Publications, New York)
   
   Additional information may be found in the [6]Mysticism Reading List.