Yeah, its fascinating stuff. For example, all of the head honchos in the nazi regime thought they were reincarnations of historic personalities; Himmler thought he was Fredrick the Great (this is actually pretty well documented) and Goering thought he was Tiberius. The book says that Goering even had a villa on the island of Capri, like Tiberius, the preverted Roman emporer during the life of Christ.
As for Hitler, he is associated w/ a character from the Grail legends, whose name escapes me (Klingsor?), but who was a sorcerer from Sicily or southern Italy, arch-enemy of Parsival and the Grail knights.
The story's primary narrator, Stein, also believes he, Stein, was a noble in Charlemagne's court where he did battle w/ the Hitler/Klingsor personality who was at that time (c800AD) the Pope.
While all of this sounds distinctly unchristian (reincarnation), the life of Christ is viewed as the single most important cosmic event in world history, and the "quest" for the Holy Grail, is the path by which initiates move to a truer understanding of the ultimate reality personified by Christ.
I don't take much of this stuff at face value, but it is fascinating, particuarly in light of "real" history that has a self-educated rabble-rouser taking over, arguably, the most sophisticated people on earth and starting a war that kills upwards of 50 millions in a little over five years. That's truly preposterous.