There's also the tried and true Lions' Commentary on UNIX 6th Edition with Sourc...

		dobbsbob on Sept 29, 2014 \| parent \| context \| favorite \| on: Writing a simple operating system from scratch (20... There's also the tried and true Lions' Commentary on UNIX 6th Edition with Source Code pdf floating around to understand what's going on in the workings of a timesharing OS. Nickolai Zeldovich a current MIT professor wrote HiStar OS from scratch and it's source is still available online to view how a minimally trusted system could work http://www.scs.stanford.edu/histar/

Lions Commentary PDFs are pirated from the book, which is still in print (royalties to Prof. Lions' widow).