Not a book for newcomers to network security but rather a book for those already experienced in network security who are looking for ways to take security to the next level.
Everyone needs to consider the security of their own computer and the information that it contains. When you have a network then security becomes even more important and the bigger the network the more important security becomes. Larger companies usually employ one or more staff whose job it is to look after the security of that network.
If looking after the security of your company network is your job then you presumably already have a significant knowledge of security issues and how to tackle them. Of course security is a moving target as crackers find ways to beat any particular security measures. That's where this book comes in with information for someone with a reasonable level of knowledge of network security to build on that knowledge and beef up the security of their network by implementing specific security features.
Introductory security books deal with what sorts of things you need to secure against and what you can use to implement that security. They don't go into great depth as to how those security measures work or what you can do to improve on them. This book takes over from where those books leave off and is an extremely useful addition to the bookshelf in any company's security department..