Even though I'm not an "A Course in Miracles" follower, this book is really a gem. Although this book is directed to Course students, it explain very thoroughly and rationally the relation between Ancient Christianity, Platonism, Neo-Platonism and Gnosticism. There is only one small flaw in all of this: it looks at "A Course in Miracles" as Jesus' original message, and hence he views Gnosticism very close to Jesus' original message in its metaphysical level. It is very interesting that he looks at the Gnostic's own mistakes as those which Course students frequently fall into.
Other than that, he chooses Bible translations very well; he chooses the appropriate Bible scholars, he chooses good authorities on the issue of Gnosticism, he shows the pertinent passages of philosophy authors (Plato, Aristotle, Philo, Plotinus), and shows the historical, social, political, religious realities of Christianity, Ancient Philosophy, Gnosticism and how they are related to each other.
I have an MA in Philosophy, and not a scholar. However, although I'm not a scholar, I would dare say that it is a reliable source to know these ancient movements.