It depends what distro you have... I have 'Hacking Linux Exposed' by Brian Hatch, James Lee,and George Kurtz, it shows you how to protect against common hacker attacks, and the more serious ones too (http://www.osborne.com)
Another good book I have is the Red Hat Linux 7.2 Bible - Unlimited Edition, by Christopher Negus, the book is a general tutorial/explanation/admin guide to RH 7.2, They have them with other distros too. (http://www.books24x7.com).
Hope this helps you out