The following books are highly recommended to introduce and support understanding of the teachings...

Entry into the realm of reality, The Guide
Translated by Thomas Cleary
Written by Li Tong Xuan
And published by Shambala Press

On aspects of the Diamond Approach by A.H Almaas:
Essence, The Diamond Heart Series, Pearl beyond price, Facets of Unity [details, buy-english]

On Super Ego:
Soul without Shame by Byron Brown [details, buy-english]

On the Enneagram:
The Spiritual Dimension of the Enneagram by Sandra Maitri [details, buy-english]
The Wisdom of the Enneagram or Personality Types by Don Richard Riso & Russ Hudson [details, buy-english]
The Enneagram by Helen Palmer [buy-english]

The Lataif:
audio (mp3), original teaching by Faisal,only available if you are an attendants of the Diamond Work [order]