Notice these books are free because they want to sell you their products, obviously... But many of them are really interesting because they show you how working the Next Generation Firewalls, how the Modern Malware is evolutioning, how we can avoid attacks like Distributed Denials of Firewalls, etc...
I've created a recopilation of many of them...
Click on the pictures to download the books. If some links are broken, please tell me.
Definitive Guide to Next-Generation Threat Protection FireEye
Intrusion Prevention Systems For Dummies Sourcefire
Modern Malware for Dummies by PaloAlto Networks
Next Generation Firewalls for Dummies by Palo Alto Networks
UTM for Dummies by Fortinet
DDOS for dummies by Corero
Network Security in Virtualized Data Centers by PaloAlto Networks