The Bahir: Illumination, by Aryeh Kaplan, is one of the legs in the three legged stool of Kabbalistic books. Along with the Sefer Yetzirah, the Book of Creation, and the Zohar, Jewish mysticism is more or less based on these works. The Bahir is an early work, and in it can be found most of the ideas that would fully flower in later Jewish mystical traditions, including the "Tree of Life" and the Sephirot.
The layout of the book is challenging. Kaplan’s translations and commentaries are together, but to read the Hebrew text, which is vital, you have to flip to the back of the book. I photo-copied the Hebrew text, so I could follow along.
But this is petty concern. We are lucky to have this English translation and commentary. For one interested in expanding his or her Jewish religious literacy, this book is essential.