Some of his other books offer a more popular, less theologically oriented introduction to his ideas. No Boundary, A Sociable God , or The Spectrum of Consciousness, Grace and Grit, by Ken Wilber Wilber presents a developmental stages approach to understanding personal and transpersonal growth. If you prefer a psychological approach try the first book, for a sociological approach try the second. Quite readable and very thought provoking. The book contrasts this view with Wilber's.
Some chapters are quite technical and difficult to read but there is some very interesting chapters.
I also have a summary article if you're interested and a response by Wilber. An attempt by a Tibetan religious leader to describe a humanistic nonsectarian approach to the contemplative life. Don't be put off with the image of the warrior until you have read how he defines the term. Very interesting metaphors and has the virtue of being short. The Choice is Always Ours by Dorothy B. Phillips, et.