These documents are also available as rpms and pdf files in the RPMS and pdf
Introduction to System Administration
System Administration Guide
Step by Step Guide
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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.