If you are looking for a reference to the standard library, this is it. There is a chapter for each header. Each chapter details all functions that are available and how to implement them. The book is about 500 pages, but all chapters are useful. You won't find any fluff in here.
Fun to Read!
It just had to be said. Besides being thorough and authorative, it's fun to read! (Assuming that you enjoy programming.)
The Definitive Book on the Standard C Library
This outstanding book integrates three treatments of the Standard C library: The straight text from the standard, a commentary, and a prototype implementation. All of these show Plauger at his best.
Even though the implementation does not always look elegant (and unfortunately is not open source), it provides a high quality implementation of each aspect of the standard and is extremely instructive (I found the stdio formatting code and the math routines particularly interesting).
