The Everything Enneagram Book (2007) by Susan Reynolds.
The Enneagram in Love and Work (1995) by Helen Palmer.
The Enneagram: Understanding Yourself and Others in Your Life (1988) by Helen Palmer.
The Essential Enneagram (2009) by David Daniels and Virginia Price – this is the best “little” book, providing a wonderful summary of key features of each type.
More academic / technical
The Spiritual Dimension of the Enneagram: Nine Faces of the Soul (2000) by Sandra Maitri.
The Enneagram of Passions and Virtues: Finding the Way Home (2005) by Sandra Maitri.
The Enneagram: A Christian Perspective (2001) by Richard Rohr OFM.
While not without some degree of criticism, in my experience The Enneagram is a wonderful tool for growth and change; for working with what Jung call’s our shadows and our pathologies for the sake of health and wholeness. If affirms strongly the gifts and richness of difference, while at the same time aiding in ones quest for self-awareness, consciousness and relational health. Sadly I’ve also seen it used badly – a tool for self-justification, blame, avoidance, and manipulation etc.
From a Spiritual Direction point of view it’s a really important too. Really useful too in relationship-counseling and such like.