Securing & Optimizing Linux - Free pdfs that you can read that will show how to secure and optimize RedHat or Linux in general.
Linux From Scratch - LFS is a project that provides you with the steps necessary to build your own custom Linux system.
Advanced Bash-Scripting Guide - An in-depth exploration of the art of shell scripting
Site includes Bruce Barnett's tutorials on UNIX shell programming and various other arcane subjects.
Shell Quotes - How to use quotes in a shell script.
Regular Expressions - Written for Solaris, so your milage my vary..
SED - The Stream Editor - Editing a data stream.
Bourne Shell - Everything you wanted to know.
AWK - The basic power tool for UNIX.
Introduction to Computer Science and Java Programming at Princeton.
Thinking In Java, by Bruce Eckel. TIJ is free (hard to beat that price) for download.
Randal L. Schwartz Columns - Randal L. Schwartz writes a monthly column for Linux Magazine, a monthly column for the Perl Journal, and a bi-monthly column for SysAdmin Magazine. New columns are placed on-line according to the agreements he has made with them, generally around three months after they've appeared in print.