Google - to find the best and worst solution, but which is which?
Generally when you attempt something on Linux you will Google and then try and filter through pages till you find the `best' solution. But what gaurantee do you have that the information is written by someone that really know's their stuff? Read on...

Distribution Specific Documentation
Each distribution generally comes with comprehensive administration and configuration guides. The advantage of this documentation is:

• It's usually well written
• It has helpful good quality accompanying diagrams and graphics
• It answers the "What is the proper way of doing X on distribution Y?" question

Ubuntu
For Ubuntu if you go to http://ubuntu.com and click Support and then Documentation you get an index page leading to the documentation of all the different versions.

For example: If you wanted to configure Ubuntu 8.10 email services the documentation is available here

I find the distribution specific administration documentation to be straight forward and simple to follow because it's not written in a distribution agnostic style.

CentOS
CentOS has a similar range of documentation

Locally Installed Documentation
If you run your package manager you will probably find the above-mentioned online documentation available as a locally installable package as well. Once you have installed the documentation it will be usually be located under /usr/share/doc/*