Linux Network Security (Administrator's Advantage Series) by Peter G. Smith, Red Hat Fedora Linux 3 Bible by Christopher Negus, Network Security Hacks, by Andrew Lockhart are the best book for Linux ,networking and security.
You can also try Linux Network Administrator's Guide as a beginner.
I would add Real World Linux Security by Bob Toxen to the list. Also, I agree with crucial - nothing beats hands on. Check sites like www.webhostgear.com, tss's site: www.eth0.us, securityspace.com, gulftech.org, securityfocus.com. Then grab either a VPS or a cheap dedicated and jump in and get your hands dirty. GL!
I'd whole-heartedly recommend the excellent books by Morgan Kaufmann!
I have many of their books myself, and even as an advanced reader I use them frequently.
Extremely professional and well written. Through would almost be an understatement, quite academic in its style...