I would agree w/ that assesment simply because most of the information cannot be sourced. You must remember that the essential element of a secret society is secretiveness.
When he says, for example that Dietrich Eckart(sp?) was a poet that initiated Hitler into satanic rites, evidence that Eckart, a real poet known to associate w/ Hitler, was a satanist would be hard to come by.
The majority of his info concerning Hitler came from a man named Walter Stein, who was something of a mystic ( a Rosicrucian), and who claims to have known Hitler in his vagabond Vienna days. It is Stein's view of Cosmic Time, stretching back to Atlantis, that makes up the book's foundation. You simply can't source that stuff. Ravenscroft readily admits that the story he is telling is impossible to prove using standard historic models.
Where Ravenscroft does cite hard sources; those check out. And I believe it is these sources, such as Einhard (Life of Charlemagne) and William Shirer, Nietzsche, and Mein Kompf, and various Grail legends, that prove his story has some validity.
Naturally, as a "mystic" history it will not satisfy academic historians. Conversely, can the academics even approach the mystic reality that adherents to such a society believed if they themselves don't take the subject seriously?
When a hard-headed realist like Albert Speer states that at his first meeting w/ Hitler, it was like he came under a spell, is that just a turn of phrase or something more? Likewise, nearly every Hitler biographer, from Toland to Bullock has made reference to Hitler's "demonic" ability to control people. Was that simply attenuated charisma?<P. I don't know, nor do I think anyone ever will know the complete story, but I think Ravenscroft has something interesting to add.
One's analysis of the influence of the occult on the Third Reich will obviously be influenced by one's acceptance of the reality of the occult. Also, as you said, hard evidence of occult practices (by definition secret) will be hard to come by.
The book does sound interesting.