Books on the Unix API
In-depth discussions of the Unix system and its API are important not only for writing powerful user-space programs, but for also understanding the responsibilities of the kernel.
Stevens, W. Richard . Advanced Programming in the UNIX Environment. Addison-Wesley, 1992. An excellent if not definitive discussion on the Unix system call interface.
Stevens, W. Richard . UNIX Network Programming, Volume 1. Prentice Hall, 1998. A classic text on the sockets API used by Unix systems.
M. Johnson and E. Troan . Linux Application Development. Addison-Wesley, 1998. A general overview of the Linux system and Linux-specific interfaces.