My memory is that the manuals and papers where missed off the first edition CDROM and so where
published as postscript on the net (sorry if I am wrong, my memory is rusty here).

The first edition documents at still on the toronto archive:
http://www.ecf.toronto.edu/plan9/

Bottom of the page, together with some other bits and pieces for history buffs.

For the seccond edition there is always the wayback machine...

http://web.archive.org/web/19970402233724/plan9.bell-labs.com/plan9/vol1.html

-Steve