#OOP?
The most comprehensive overview of the MACH operating system is contained
in "The NeXT Book" – published by McGraw-Hill (I think, my copy is
elsewhere). Otherwise, there are several articles in Unix Review, UseNix
Proceedings, and a slug of technical reports available from CMU. It's an
interesting OS, and some of the ideas contained in MACH are creeping into
other systems. Sorry for the delay in replying, I've been up in British
Columbia, and delibrately avoided taking any form of keyboard or modem with
me.
Tom G.