It's not completely without merit, but has to be read with caution and discernment. There are some nuggets of insight in here, but there's a lot of dubious supposition and false premise too, cloaked in withering piety and holier than thou pontificating.
Again, it's not worthless, but you'd better know your subject before reading this, which begs the question: why read it?
The authors are not shy to pronounce with considered authority on subjects they know little about. They make the arrogant error of presuming that their model is paramount and all others must be twisted to fit it, never once considering that they might be better examined in their own right as an alternate model. For example, their references to Norse and Celtic mythic themes are insulting in their presumption and vacuity. They also buy wholesale into the 'alchemy is a spiritual metaphor' error (a notion that no actual alchemist ever shared) and distort it to fit their own model.
In short, this is a book which may be good to stimulate a notion or two if you paradoxically have progressed enough in your own right to know when this is wrong.