1 Unix/Linux/Posix Books

There are many good books on system programming under Unix/Linux/Posix.

Kerrisk, M. (2010) The Linux Programming Interface, No Starch Press.
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Stevens, W.R. , B. Fenner and A.M. Rudoff (2004) UNIX Network Programming: The Sockets Networking API, Pearson.
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Stevens, W.R. (1999) UNIX Network Programming: Interprocess Communications, Pearson.
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2 Unix/Linux/Posix WWW Pages

There is a lot of information available on the WWW on system programming under Unix/Linux/Posix.