zlacker

This course you build a small OS with the seL4 microkernel on a sabre lite board http://www.cse.unsw.edu.au/~cs9242/current/

Of course there's also Lions' Commentary on UNIX 6th Edition, with Source Code by John Lions and the plan9 operating system design specs you can go through to see why they did what they did. They're "old" resources but basics have stayed the same.