Rating: 

This is a good book (quick reference) for userland developers. I liked his approach.

In my opinion, he could improve his book going deeper on some examples, adding sockets chapter, mix more userspace and kernelspace.

I agree with last comment too, there are others books around this topic that could explain more in details some topics.

In the Bibliography section, the author did not include two of the most important books that cover related material: 1. "Advanced Programming in the UNIX Environment," by W. Richard Stevens, and 2. "Programming with POSIX Threads," by David R. Butenhof.