The Book of Changes (also known as the I Ching or Yi Jing, depending on your favorite Romanization). Explaining what it is would be too lengthy here; it is better to start with a précis like the Wikipedia article, and then (I would say) to the introduction to the Wilhelm-Baynes translation by C.G. Jung. One can actually employ the divination rituals, which often yield "answers" which are often strangely relevant and sometimes shocking. I came to the Book of Changes via another book, _Psyche and Symbol_, by Jung, which I found very difficult. At the time I was 17 or 18 (so, late 1950s) and "we" were all going through an End-of-History moment when all relevant, important facts were supposed to be known and everyone was supposed to be well-balanced and rational. (This is so remote from contemporary cultures that I guess you had to be there to understand it.) It was "the old folks' home in the college" as the prophet Zimmerman put it, dry and asphyxiating. I wanted to break out of that. Jung's preface to the Book of Changes was included in _Psyche and Symbol_ and instead of being confused with his rather dense mythographies elsewhere, the part about the Book of Changes was very clear and included a description of the material practice. So I ordered the book (actually, in those days, two volumes from the Bollingen Foundation and far from a mass-media or hip item) and started using it immediately. Its style was not at all like the Bible, the Qur'an, or most of the other spiritual works of fame; I suppose some of it resembles the Daode Jing (Tao Te Ching) but since one works actively with it -- it requires interpretation -- the experience of using it is unlike that of any other books I know. It certainly changed by life by changing the perspectives from which I looked upon things. I had sort of escaped into a wider and stranger world. (Curiously, I later became a computer programmer, using from time to time the binary arithmetic supposedly invented by Leibniz under the inspiration of the Book of Changes, although this may be in the area of legend.) There are quite a few editions of the book online, some of them bilingual in case you can read ancient Chinese or you like the décor; but an investigation of the characters and their etymology may also be relevant.
Feng and English rides around with me in a messenger bag, but I derive joy from reading other translations, too. It feels like kind of a living document, with everyone viewing the underlying ideas from their own perspectives.
