UNIX Network Programming, W. Richard Steves, Prentice Hall, 1990.
Linux Threads Frequently Asked Questions (FAQ), Sean Walton, January 1997. Available at http://sunsite.unc.edu/pub/Linux/docs/faqs/Threads-FAQ/html/.
The LinuxThreads Library by Xavier Leroy is an implementation of the POSIX 1003.1c thread package. It provides kernel-level threads, using the clone() system call. Available from http://pauillac.inria.fr/~xleroy/linuxthreads/. Red Hat users may already have this installed.
What is Multi-Threading?, Martin McCarthy, Linux Journal, Issue 34, February 1997.