This has been an abiding interest of mine for some years, although I realize that my interest is an obscure one. Most people attribute it to Leary, although he made it clear (sometimes) that Brian Barritt was also involved. Robert Anton Wilson wrote the most about it, although to me Leary's more terse version holds more appeal. Antero Ali wrote entertainingly about it but para-theatre is not my thing. Karl Jansen realized its real potential-- as a model for describing drug experiences. Shulgin had a three point scale but that actually just described how 'out of it' you were.
It looks as if the writer David Jay Brown in his upcoming book may add more to this subject. An interview with him, Alli and others gives some thoughts on the model in Boing Boing., with reference to Robert Anton Wilson.