This is the best introductory book you will find on the Enneagram. Wagner's guide is a clear and concise introduction to the Enneagram, useful for personal exploration and as a teaching ID for workshop presenters and counselors.
This comprehensive book with charts, exercises, and bullet descriptions, yields an experiential understanding of basic Enneagram principles such as:
Authentic values and their personality substitutes
Resourceful and non-resourceful cognitive, emotional, and behavioral schemas and how they shift under stressful and flow conditions
Developmental influences
The three centers of sorting and deciding
The defense mechanisms, principles and paradigms, virtues, passions, and both healthy and maladaptive instincts of each of the nine Enneagram personality types.