I am generally wary of typologies, not wanting to categorize myself or others in ways that get in the way of seeing people as they are. That said, i have heard good things about The Enneagram for many years, and as i've been slowly reading Facets of Unity (recommended by Lucy Perry), i have become more interested. When i mentioned it to Carol Placer, she recommended:

 

The Essential Enneagram, David Daniel (to help figure out which type you are)

The Spiritual Dimension of the Enneagram, Sandra Maitri (??)

The Wisdom of the Enneagram, Richard Risso and Russ Hudson (with to-do advice for each type's struggles/learning edges)