These documents are also available as rpms, tarballs tarballs and pdf files
Installation Guide - x86, itanium, x86_64, em64t
Installation Guide - s390, s390x
Introduction to System Administration
System Administration Guide
Reference Guide
Security Guide
Step by Step Guide
Clustering Guide
Global File System 6.0
Developer Tools Guide
Note: The above documentation is provided (and copyrighted) by Red Hat®, Inc. The documentation is released via the Open Publication License (V1.0 or later). Red Hat®, Inc. has added the further requirement that "Distribution of substantively modified versions of this document is prohibited without the explicit permission of the copyright holder." Here is a copyright notice in it's entirety.
The CentOS project redistributes these original works (in their unmodified form) as a reference for CentOS-3, because CentOS-3 is built from publicly available open source SRPMS. We don't modify the above files in any way to be compliant with the upstream vendor's distribution policy.