You will find several free guides at The Linux Documentation Project, guides page. This is a short list i pick from there, but you should scroll through the page to find what you need.
- Introduction to Linux – A Hands on Guide; Jun 2008
- Linux on the Road; Nov 2005
- The Linux System Administrators’ Guide; July 2005
- Advanced Bash-Scripting Guide; Mar 2009 (yes, this is good for understanding linux too)
There are some starters at the Linux reviews beginners page.
Finally, this is a small book available online — Linux Kernel in a Nutshell. Its in PDF form of the 2007 edition.
You should use these references only as a feeler to start you own search for things that you need.