Discovering Your Personality Type by Don Richard Russo and Russ Hudson was my capitulation to finding out about the Enneagram. Several friends rave about it, and my housemate has been to several training institutes. So I caved. Interesting book, a bit rah-rah and "buy our books" for my taste, but having taken their instrument and looking at my office through the Enneagram lens, it looks useful.
Here’s their website: Enneagram Institute: Enneagram Testing & Training.
And if you’re curious: active five with a four wing.