Network security consists of the policies and practices adopted to prevent and monitor unauthorized access, misuse, modification, or denial of a computer networkand network-accessible resources.
You can refer book for Certificate in Network Security.
Book Name-Network Security Essentials Applications And Standards
Author Name-by Stallings
Price-Rs.500 (approximately)
Link : http://www.amazon.in/Network-Security-Essentials-Applications-Standards/dp/8131716643?tag=googinhydr18418-21&tag=googinkenshoo-21&ascsubtag=fd802556-af93-46d2-85f1-95599dffa27b