I'll quote senshin from that question:
I'd try and read up on this myself, but, you know, I'm not an Operating Thetan, so this stuff is hard to find.
Scientology material is really tough to find. Some is out there, though the Church denies that some of it is true. Much of what we know about it is through a small number of sources, many of them former Scientologists.
To answer senshin's question, I used material from the Fishman Affidavit, testimony given by a former Scientologist who turned against the Church. Wikipedia has some good information, but it's based only on a handful of sources, many just like the Fishman Affidavit. The Church denies that much of it is true. Only top members can learn most of the important stuff.
So what sources can we use - or, rather, are we willing to consider testimony by ex-Scientologists, which is really the only thing we have, canonical information?
See also, of course, What are good sources when answering questions?.
This seems to be an interesting starting point.