I've been studying the Enneagram in the context of personal growth for about 10 years now. These days, my go-to book is Deep Living: Transforming Your Relationship to Everything That Matters Through the Enneagram by Roxanne Howe-Murphy.
Have you used the Enneagram or another personality typing system for personal development? I recognize that different systems work well for different people/purposes. I like talking about the Enneagram every chance I get.