Operating Systems: Three Easy Pieces
Apropos of nothing, if you want a VAX/VMS system to play with (they profile one in the book) let me know :-)This is a great book btw and you can't beat the e-price of free. I also recommend "Operating System Concepts" which is sort of the canonical book on the topic, or if you can find it a used copy of the Springer-Verlag book "Operating Systems Design".