[AimHere]

As well as Minix, which is intended as just such a teaching aid and comes with a textbook, there's the Lions book, which is a complete annotated source listing of an early Unix, and which was passed around as nerd Samizdat for a while, until the Unix copyright status was changed.

http://www.lemis.com/grog/Documentation/Lions/index.php

[Waterluvian]

Thank you, and thank you everyone else for all these resources. My wife will be rather displeased that I'm not playing Divinity 2 with her this holiday weekend ;)