The best, I think , is this C tutorial . The comp.programming.threads FAQ is also available for your thread programming needs. MMX programming in Linux is explained here. Also available is the Linux Programmers Guide . Extensive introduction, laid out the way you'll need to use it, in html now.
Socket programming is very hard to pick up from man pages and source code alone.
There is no other site as good. This page is fantastic. Totally clear, gets right to the point, doesn't wiffle waffle with tiddly details you don't need to be bothered with. The Perl Info Page. Contains a tutorial on using perl in the shell and for CGI web stuff.