My recommendation for the serious student and practitioner of I Ching (Yi Jing) is to get your hands on:
The Complete I Ching: The Definitive Translation [1998] by Taoist Master Alfred Huang
As this tome's title states, it is the most definitive and exhaustively thorough reference about I Ching that I have worked with regarding the I Ching trigrams, hexagrams, linear, and yin - yang relationships. Having at least a fundamental knowledge of Lao Tzu's (or Laozi's) Tao Te Ching (written circa the 6th Century BCE) will greatly assist you in further understanding the philosophy behind I Ching and how it works as a divination tool.
