[CentOS] Recommendation/pointers please - Need to brush up on CentOS/Linux command line tools

Hi all,

I would very much appreciate any suggestions on any online resources, or 
even a decent book to purchase with the focus of brushing up on Linux 
command line tools. The focus is on troubleshooting type commands, 
adding users from command line
 and so forth.

While I am reasonably comfortable using the command line, I will be the 
first to admit I am slow on some things and have gaps in my knowledge 
especially for those commands I have never needed to use. I tend to use 
Webmin for a lot of things. For an upcoming job transition I need to 
fill in the gaps to be able to do these same things I currently do via 
webmin at the command line.

So any suggestions/pointers, etc would be very much appreciated. I 
wonder if there is a Linux for Dummies book out there (provided it deals 
with the command line vs. a GUI).

Thanks

Dale