This revised guide to Unix networking APIs updates coverage of programming standards and debugging techniques, and covers operating systems including Red Hat 9, Solaris 9, HP-UX, Free BSD 4.85.0, AIX 5.x and Mac OS X.
This is the definitive sockets API programming book and it's a good example of a solid text on low level software development. There's a few anachronisms that should be cleaned up, but those are kind of neat historical artifacts that remind you that there are various flavors of Unix out there.
It is what it is. Thorough coverage of the Sockets API. In C, of course, but applicable to the networking API of many other languages because they are all built on top of the C API.