Advanced linux security

[Red Squirrel]

I'd like to learn more about advanced linux security, so more about permissions, firewalls, and even beyond that. I just feel that while I think my servers are secure, there may be things I'm unaware of, that make them unsecure. I'd like to read up on the subject more deeply.

Could someone recommend me books/sites I should check?

 

[SagaLore]

Get the first Hacking Linux Exposed book. Learning security is kind of dry until you can see real world examples.