resources for system level security?



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.