This book is a mixed bag, there a moments when I think even a orthodox Catholic might be able to agree and then there are points when reading this book that one gets the sense that an orthodox Catholic would either call this New Age Mumbo-Jumbo posing as intelligent, as soon as certain authors tried to make certain Saints and even Jesus and the Blessed Mother into pagan myths re-embodied I saw where this was going and so my review for this book is mixed because of that, although to be sure this book is subtitled, "Christianity and the Perennial Philosophy" not how Christianity is the Perennial Philosophy, so rather than trying to, "draw all men unto Christ." (cf. John 12:32)
The book seems to tell all men to stay where they are and not bother leaving since they are all part of the same thing anyway, this book sorely goes past the fact that either Jesus Christ is God or He is Not, and if He is, no other religion - no matter how much The Spirit may have informed it - can compare to the One He Founded, and some contributors in this book would agree, but even then they attack the Post-Vatican II Catholic Church, and so show themselves to be a mixed and odd bunch, ergo my mixed review.
If you do read this, and are orthodox you will need a discerning eye to know what is agreeable and what is not.