This is THE guide to UNIX network programming APIs. Whether you write Web servers, client/server applications, or any other network software, you need to understand networking APIS-especially sockets in greater detail than ever before. You need UNIX Network Programming, Volume 1, Third Edition. In this book, the Authors offer unprecedented, start-to-finish guidance on making the most of sockets, the de facto standard for UNIX network programming with APIs - as well as extensive coverage of the X/Open Transport Interface (XTI).
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.