I strive to avoid use of words like “soul” and “spiritual” because the common meanings have supernatural connotations. It’s like Einstein’s use of the word “god”—when one uses these words, they are open to misinterpretation by people putting their own definitions on them. I don’t think any two people have the same definition of “god” let alone “soul.” I don’t know how many arguments I’ve had with theists that want to tell me Einstein believed in their version of god, because he used that word to mean natural law. Ah, the argument from authority, how it chafes!
I have a unique perspective on some of this stuff: in my misspent youth, along with seeking god, the supernatural, the spirit and all that numinous stuff, I also experimented with illegal substances (and hopefully talking about this isn’t a violation of the rules here; it is relevant to my point, is all). When I was a kid, I voluntarily went to church to see what all the fuss was about and experienced the “still small voice” during a particularly emotional church service. I was all of eight. Under the influence of the mob mentality, I started crying in joy and allowed myself to be “saved.”
Within six months, I realized the god thing was nonsense and quit. But I still sought answers, so I turned to the occult, then drug experimentation.
Under the influence of LSD, methamphetamine, and ecstasy, I experienced exactly the same feeling as I had during the “still small voice” episode. It is obvious to me that it is all brain chemistry driving these things. An ecstasy rush feels just like religious ecstasy, except for the fact that it is actually stronger and longer-lasting than the religious epiphany.
During my meth days, I experienced an NDE as well as sleep paralysis several times. The NDE wasn’t even brought about by physical trauma of any kind; I just did too much speed, went to lie down, and then it seemed I left my body and was floating by the ceiling, then I was sailing off into the darkness, horrified because I didn’t have a mouth to scream with. it took me a while (and getting clean) to figure out what had really happened and that it wasn’t anything supernatural or having to do with a soul or spirit. it was very real, and left a strong impression on me. I have a journal from those days that reads like a manual of psychosis, even describes the NDE and the episodes of sleep paralysis.
Since pilots experience similar things during high stress training and high velocity maneuvers in jets, and going by my own experiences, I am certain that the phenomenon described in the book mentioned in the OP can all be explained mundanely, by changes in brain chemistry and physiology.