I found most interesting part of this book the arguments and research into the parallels between vedic and vajrayana pantheons, and the claim about transmission of Indian soma to Vajrayana amrita. The author has impressive amount of knowledge of his subject, the exposition of various myths and iconographic features is extremely detailed. It seems to me that something quite psychedelic probably was ingested in vajrayana initiations. But, if the soma/amrita was a psychedelic mushroom, and large scale psychedelic mushroom use was a norm in early vajrayana, one would expect to find a bit less ambiguous and stretched evidence for these claims than the author is able to provide.
The overall history bits and comparative mythology between vajrayana ja earlier Indian religions, and tantric practices was the most solid parts of this book. For the rest 200 pages the author is repeating the same fixated "insight" that everything in boddhisatva images with a vague resemblance or even isolated features like red and white colour refers to psychedelic mushrooms. What the thinker thinks, the prover proves. At least I was not convinced that every boddhisattva, dakini, parasol shape, red and white thing etc. would be a hidden reference or personification of psychedelic mushrooms. Also there is great ambiguities in the argument, as the author at the same time insist that soma/amrita was psychedelic mushrooms but also brings in a variety of other possible candidates. I think based on his own evidence and the mass of ambiguities and unsolved puzzles in them, the author should be much less convinced of his own hypotheses and conclusions.
Also I find it a bit anachronistic that we would need a pharmacological reason for every alteration of consciousness or trippy feature in spiritual traditions. Placebo is a strong drug, and definitely there are other ways than psychedelic substances to alter your consciousness.