There are many useful books on the enneagram to look into. Helen Palmer’s The Enneagram is a good place to start. Another good book is Personality Types by Don Riso. For a Christian (mystic) perspective, Richard Rohr’s Experiencing the Enneagram is the best. For a deeper, more psychological understanding of it, Ennea-type Structures by Claudio Naranjo is a very good book. It is very useful for understanding the passions of the types and he also gives useful pictures of each type, which displays some of the physiognomy that accompanies type and the enneagram. He has another book called Transformation Through Insight that evaluates famous types and examples in literature, which is also illuminating. Another more esoteric good book, but closer to the source of the enneagram is In Search of the Miraculous by P.D. Ouspensky. The homepage discusses the enneagram’s origin through the great mystic George Gurdjieff. In the book, one of his pupils, Ouspensky, describes years studying with the master. Gurdjieff’s teachings cover the enneagram and the laws that create it throughout the book. He refers to it simply as the law of types and calls it a man’s chief feature or weakness.