Hameed Ali, under the pen name A.H. Almaas has written more than 20 books each of which is highly highly significant and a contribution of the utmost value to the knowledge and understanding of humanity's spiritual nature and spiritual potential.
Here I recommend three which are relevant to the journey.
-The Inner Journey Home (Parts 1-4). This is the book that gives a detailed description of the path and its fruition. Particularly relevant is the understanding that the human individual is a soul; a living organism of Being.
-The Unfolding Now. This book illuminates the path's method to reach the primary awakening to ones nature and identity as essential presence.
-Space Cruiser Inquiry. This book explains and gives examples of Diamond Approach's central practice of open ended inquiry.
-Diamond Heart book 1. This book elucidates some of the basic concepts of the path.
For those who would rather listen than read the suggestion is: The Diamond Approach available from SoundsTrue.com. It is lengthy, but an highly illuminating interview with Hameed, the founder of the Diamond Approach.
Here are two video suggestions: