resources for system level security?

최봉환 <[email protected]>
Newsgroups gmane.comp.security.penetration
Message-ID <[email protected]> 
Hi all, 
I started to work of focusing on linux system level security. Mostof the servers are providing web services. 
Although I have been working on application pen testing, I havelittle experience to handle with security issues of system or OSitself.
Could you recommend where I should start for it?
(useful books, web sites, or concepts/terminology I have tounderstand)
Any advice would be highly appreciated.
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