Lon DuQuette, Deputy Grand Master of the American Caliphate Ordo Templi Orientis, has a unique take on the subjects that he writes about.
Unlike many authors (particularly many of those who write about Kaos or "pseudoreligions"--cf. Robert Anton "Bob" Wilson), DuQuette manages to be humerous without being a cynical relativist. His book "Magick of Thelema" is probably one of the best starting points for people who are interested in learning about "orthodox" OTO-style Thelema. Not being an "orthodox" Thelemite, but rather "heretical" (though I am an Assosciate of OTO), I don't entirely agree with all his points made in the book. What I like THAT one for is his genius for bringing together all the bigger rituals of Crowley's make and commenting on them in what I perceive to be more of a suggestive--rather than authoritative--manner. For the more "orthodox", however, the book is definately worth your while.
Essentially, DuQuette does the same thing with this book. My first introduction to Qabalah was Dion Fortune's classic, "The Mystical Qabalah". I still reference it when I need to figure something out. While a profoundly influential book, it certainly has its own flaws. One technical error of the book is the lack of any discussion of the paths on the Tree--thus an entire half of Qabalah is pretty much ignored by her, and arguably the more important half. A second flaw is style--comparing aspects of the Qabalah with current science, she sometimes sounds loony due to the fact that her comparisons are to discredited ideas (though they would have been useful for those who were current in the time, and her point still stands albeit it's not as forceful). The second major stylistic flaw with her book is--it's boring. Horribly, horribly boring. Her style is more academic there than friendly. She sounds like a professor describing some obscure aspect of Newtonian physics (of course, the book IS instructional material, so that fits).
Here is where DuQuette's book comes into play. His book is much easier to read through than Fortune's, and just as informative. It's also more current, which might be helpful in some ways.
To those who would say that only the Jewish Qabalah is accurate, I say "bull". Modern scholars have made a convincing case that the Qabalah originated with the Greeks, not the Jews. Anyone who actually uses Qabalah knows that either way it is a very flexible system. Anyone can create gematric systems if they know how, and the structure and meaning of the Qabalah is unique to each person. It is not some racially-secluded attavistic, nationalist philosophy. It works with the Bible, it works with Thousand and One Arabian Nights. Heck, it works with Alice in Wonderland. Learn it, use it, love it.