Linux documentation is present in form of Guides, Specific topic how-to's, man pages (Manual Pages), info pages. All these found at one site,
http://www.tldp.org/.

This site also features two excellent magazine sources which can be viewed online.
Linux Focus Magazine.
Linux Gazette

Here are few references for starters:
A nice introduction to Linux.
http://www.tldp.org/LDP/intro-linux/html/index.html

The Unix and Internet Fundamentals HOWTO in non-technical terms.
http://www.tldp.org/HOWTO/Unix-and-Internet-Fundamentals-HOWTO/index.html

For those who want to Switch from DOS/Windows to Linux.
http://www.tldp.org/HOWTO/DOS-Win-to-Linux-HOWTO.html

Frequently Asked Questions on Linux.
http://www.tldp.org/FAQ/Linux-FAQ/index.html

Some excellent books which you can keep with you as reference.

Though not essentially a Linux only book, Unix in Nutshell covers all important aspects of tools required to survive on Linux (Developers). One of the best hands on refernce books I have seen.
Unix in a Nutshell - Orielly Publications.