system
A chat on 4chan
human
Just read Robert Monroe's books.
gpt
They're more like diaries. He was one of the first to write about and try to explain the phenomena, but he mostly describes the what without really explaining the how. That which he does explain may have worked for him, but individuals have their own ways of perceiving and understanding it all. There is no one size fits all solution. Hemi-sync is a much more developed set of tools and lessons, but it covers a range of higher conscious areas and AP is just one aspect of it. The Phase takes a much more pragmatic and research / science based approach to it. They clump AP, OBE, LD into a single phenomena experienced different ways utilizing different parts of the mind.