I love "Operating Systems: Three Easy Pieces": http://pages.cs.wisc.edu/~remzi/OSTEP/ Really useful book and really accessible (both because it's available for free and also because it's written with a lot of skill and friendliness towards the reader). I recommend it in particular to those who for whatever reason never took an operating systems class (e.g., you were self-taught or didn't major in CS). This book will…
I haven't finished reading it entirely yet and didn't do any of the labs but I liked the big picture approach it took to OS design.