Security has different meanings depending on context. Java's core sandbox security model was originally intended to defeat viruses and trojans. Authentication, encryption and other security models were added to provide different kinds of security. The authors explain how Java components work so they can show how they might be subverted. Without knowing what the risks are you can't apply effective security measures.
The Java security features examined include class loaders, cryptography, certificates, key management, signatures, SSL, authentication and permissions. The authors explain where and how particular security features are best implemented and explain their limitations in the real world. For example, many people routinely grant signed Java applets permission to read and write files on their system believing a signed certificate somehow makes the application safe. In practice, as anyone can create and sign a Java applet or application it proves nothing of the kind and can still be setting you up for a fall.
Java security is non-trivial. Security is an arms race in which the two sides constantly leapfrog each other. Java Security is well written with many examples but it's a fairly technical read. If you're serious about Java application development, however, you need to read it. Because you can be sure the bad guys will. --Steve Patient