For a small book, this covers a surprising amount of material. The main apple extensions are actually quite small, just a few event handlers and utilities, but the book covers everything from the directory structure of the Java framework to setting up application servers. The details of the extension classes has changed since publication, but it's a simple matter to lookup the new class names and the semantics are the same.
Just about everything you could need is here, albeit in an introductory form. The chapter on tools discusses some of the available editors and build tools but I would have liked to have seen more in-depth information on using Xcode (then Project Builder). Later chapters detail converting JAR files to OSX application form or delivering with Java WebStart.
The book deals with more than just Mac peculiarities. It serves as a guide to cross platform development in general. This might sound odd given the "write once, run anywhere" promise of Java, but there's more to it than that. The fashion for Java applications that look the same on all platforms is long gone, and the pluggable look and feel architecture has its own problems. If you develop in a windows environment, you are likely to find that your application looks nasty when dropped onto a Mac.
The basic extension mechanism given is a plugin architecture to isolate the platform specific code. This is used to good effect with a few examples of how an application is integrated into the OS X desktop. Later chapters introduce some of the more interesting APIs such as QuickTime and the Speech API.
The final chapters introduce more enterprise level subjects such as how to set up Tomcat and JBoss. JDBC development is covered with instructions on using MySQL or PostgreSQL. I can't vouch for the accuracy of any of this, but it appears to have been given a clear and concise treatment.
All in all, a very neat little introduction to all things Java on OS X.
Highly recommended if you are a Java developer (of any level) and the Mac is one of your target operating systems.