If you are serious about Linux security, you should have this book.
Free software magazine has published review of this book:
Security is one of the important reasons GNU/Linux is chosen over MS Windows. Many folks will claim that GNU/Linux just isn’t targeted as often. Could be—but it could also be that it isn’t targeted as often due to its design. SELinux takes this concept one step further. Not just satisfied with the inherent security, SELinux has been developed by a team of concerned professionals and is now included by default in the 2.6 kernel. Yes, you may have SELinux already and didn’t even know it.
There is a lot of information packed into these 456 pages. Not very many screenshots, but that wouldn’t really be appropriate anyway. Most of the examples are text files and they are displayed quite clearly. The command line instructions typically include the results you should expect from execution. This is as helpful as the instructions themselves and is done properly throughout the book.