Added UNIX System Architecture by Prabhat Andleigh.

This is a really nice small (274 pages) book on UNIX architecture.
It "reads" like K&R, has good straightforward explantions and nice diagrams
of structures and things, w/o unnessiary bulk. It covers both BSD and sysV.
I wish this had been the textbook for my undergrad OS class rather
than Tannenbam.