Good, but a little outdated. For general systems overview, as well as introduction to UNIX architecture, I'd actually recommend my textbook, which is the CMU intro systems book, Computer Systems: A Programmer's Perspective[1]. For OS theory, our OS/advanced OS textbooks are fine, but for actual implementation my coworkers recommended The Design of the UNIX Operating System[2], Linux System Programming[3], and Understanding the Linux Kernel[4].

[1] http://www.amazon.com/Computer-Systems-Programmers-Randal-Br...

[2] http://www.amazon.com/Design-Operating-System-Prentice-Softw...

[3] http://oreilly.com/catalog/9780596009588/

[4] http://oreilly.com/catalog/9780596005658/