There's plenty of online resources. Do a search on Google for the specific topic you are interested in or search the forums here or join in on the conversations in IRC or .......

http://www.linuxdoc.org/guides.html

The "Linux Administrator's Security Guide" is online at http://www.seifried.org/lasg/

Securing and Optimizing Red Hat Linux. A guide for information system, configuration, optimization and network security professionals. 727KB Download at http://www.ebookdirectory.com/software.html


There are a whole slew more!!!!