There's this http://lsub.org/who/nemo/9.pdf

and the intro to plan9 http://lsub.org/who/nemo/9.intro.pdf

and, tbh, I've read the kernel source code and it isn't that hard to follow - which is one of the goals - there's even an LXR http://fxr.watson.org/fxr/source/?v=PLAN9