I'm glad to see this. Crowley was a very accurate observer in many cases.

Henk Barendregt wrote a recent account; he's a professor of math and computer science at Nijmegen (Netherlands) and an adjunct at CMU.

The comment that this is about developing skills is very accurate. Drugs can induce similar states but they don't help to develop the cognitive control skills. Unfortunately we have very few disciplines that teach the development of cognitive self-management without a lot of peculiar window dressing.

Regarding Crowley's comment on his later experiences, Wilson Van Dusen reports similar phenomena in these investigations of the hallucinations of mental patients (and of Emanuel Swedenborg). The most valuable aspect is the phenomenology rather than the speculation about causes (though that is also interesting).

Summarizing, it looks like the brain can operate in various modes other than the ones we find "normal" and to some extent can be switched between modes by skills that can be learned through training and practice. In some cases access to modes other than "normal" ones may be helpful.