"Operating Systems: Three Easy Pieces" is a more modern book to OS concepts, explained with real source code as opposed to algorithms, and is regularly updated.
This is what I learned from, along with some xv6 labs. Just wanted to say that both Remzi and Andrea were exceptional professors and their book is clear and concise.
http://pages.cs.wisc.edu/~remzi/OSTEP/
(Online version above, you can also order a print copy).