Operating Systems: Design and Implementation

It was very early in my education but Tanenbaum's OS book, which I read while playing with his Minix source code, opened my eyes quite a bit to how Operating Systems worked.

MMURTL - now there's a blast from the past!! I learned a lot from that book (by Richard Burgess IIRC) and playing around with the code was fun. Mostly 386 assembler and some C from what I remember. Yes, the Minix book is also good.

Operating Systems: Design and Implementation - Andrew S. Tanenbaum Always a good one when you want to brush up on arcane knowledge.