This one goes back a long way, all the way to the 1950s.

The life and work of Stanislav Grof. Started out with LSD for therapeutic purposes in his work as a psychiatrist, then developed a system of intensive breathing instead, to achieve the same results when this became illegal.

My favourite book is Realms of the Human Unconscious. There is also LSD Psychotherapy (and much more).

I've written several articles about him on Medium some time ago. If you're interested I could send you some links.