This book is frequently referred to in “Alchemically Stoned” which is about the entheogenic origins of the symbolism of Freemasonry and which also looks at Mithraic symbols in Freemasonry. The current title is the other way around.
The authors of the present title are of the opinion that experiences with mushrooms and other mind-altering substances form the basis of the mysteries of Mithras. Mithras with his Phrygian hat (red and spotted) is actually a mushroom. The torchbearers stand on one leg for the same reason. Other symbols are looked at from the same perspective. The results are amusing, but seldom really convincing.
The also has large parts which have little to do with entheogens. Some theories about Mithraism are dealt with and, for example, the -to me- fairly credible ideas of Ulansey are debunked quite convincingly.