This text from 1910 seems to be an improvement and enlargement of the famous “Kabbalah Unvieled” of MacGregor Mathers (also reviewed). It isn’t that much of a thick book, but still quite large, especially for an ‘onliner’. Unfortunately Westcott decided to keep the order that Mathers used, being a short introduction to the concept of the Kabbalah, then quite detailed explanation of the Kabbalistic wordgames Gematria, Notaricon and Temurah, explanations of difficult subjects such as the Shemhamphorash and after this explanations of Kabbalistic basis-ideas such as these of the four worlds, three souls, (arch)angels, etc.
But, the text is well-written, clear, quite detailed (very detailed at times) and all in all a very good introduction to the practical Kabbalah.
Available online from the Hermetic Order Of The Golden Dawn page.