There's always a problem with technical books of this kind: they're either too vague, talking about general concepts, not really relevant past the moment you learn enough, or far too detailed, ungreppable printed man pages (hi there, UNIX Network Programming :). This one strikes a fairly balanced point between those two ends of spectrum, being an useful review of tools Posix presents at your disposal. I've read that book during my university courses, and again, between jobs. Worked like a charm, every time. Recommended, if you want to get a general idea what kind of gnomes jump up and down behind the scene in Unix world.
Note: this book doesn't describe much in terms of kernel internals and structures. For this kind of information, in Linux flavor, read Linux Kernel Development 3rd Edition.
eBook Unix System Programming