By chance I started reading this book after hearing an interview about the "God Helmet". A Canadian scientist can induce religous experiences in believers (and strange golems in non-believers) by concentrating a magnetic field at a give place in the brain. See the book "Mindfield" for more info.
So when at the very start of the book James dismisses purely physical sources of visions I thought "well, of course, you don't know what we know now, there was not the technology, not your fault".
But then he says that non believers who dismiss visions and religous experience as bad digestion, or hunger, or whatever should do the same with scientific theories. There's a difference though. Scientists reproduce their results, other scientists reproduce their results. Fakers and bluffers are eventually found out. His error in this case was nothing to do with not having the technology.
And that error (putting religous experience and the operation of science on the same level) means that the rest of the book is probably full of other gross assumptions and hence meaningless.
It may be of interest as a histotical document though.