I am looking for official documents on all POSIX standards and specifications. I would like to be able t read the docs to get a better understanding of UNIX systems and how they work at a low level. I've seen links here and there to opengroup.org, which I'm assuming is the entity behind the standards (however, I've heard IEEE mentioned a lot too).
For a good rundown of POSIX, see this question: What exactly is POSIX? It answered a lot of questions for me.