The Sony SDK is really made for game development and the stuff given there is also 100% directed to that. It’s nice to have or nice to read but not a must have at all for anyone who is not a licensed developer. If you are a homebrew kind of guy, you might want to have a look for curiousity. Otherwise you should give PS1Light a go.

There have been many leaks in the past, but most of them were not even useable. The recent 3.60 leak which occured after some drama in the scene is most likely the best one. If you are starting of with it, the docs will guide you the way. Don’t worry, real licenses don’t get much more info either.

Edit: If you are really now looking for it, try to google. Sorry but since I don’t support piracy and I can’t give links to a downlaod here.