As far as Poser goes, that spec is really the only one you need, as the fundamentals described in it really apply to the .pp2, .hr2, etc. files as they are really all structured the same with some slight variances that are obvious when looked at in a text editor.

Now you also have to realise that it is not up to date as it was last updated shortly after Poser 5 was released, so there will be some developments that you won't find in there.

Maybe I'll see if the author will allow me to put it into a wiki, so that the community can keep it up to date.  Something to think about...