As well as CentOS and Red Hat RHEL there a number of other linux distributions we recommend

Beginners

Debian

http://www.debian.org

BOOK
http://images-eu.amazon.com/images/P/0764576445.01._OU02._PE30_SCMZZZZZZZ_.jpg
Debian

Ubuntu

http://www.ubuntu.com

Advanced

Linux From Scratch

http://www.linuxfromscratch.org/
http://www.linuxfromscratch.org/blfs/
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