If you're interested in Kabbalah, by the way, avoid anything recommended by Madonna, she hasn't the slightest idea what she's talking about. The one and only man to go to is Gershom Scholem, he was Professor of Jewish Mysticism at the Hebrew University for years and basically wrote the book on the secular study of Kabbalah. His book Major Trends in Jewish Mysticism is absolutely mindblowing. It must be read by anyone who wants to understand what makes Judaism tick. He also has a fascinating biography of Shabtai Tzvi, the 17th century rabbi who most of the Jewish world thought was the Messiah. It was a moment of immense upheaval in the Jewish world, and Scholem theorizes that it had immense and far-reaching repurcussions. Its a very long book though, and tough reading if you dont know something about the subject already.