You learned a lot in this chapter. The combination of looking through the source code while reading the enhanced explanations here will help you to get a grasp on designing and implementing a document-based Cocoa application of your own.
If you are confused, reread the chapter one section at a time and concentrate on the relationships between the source code in Project Builder and the user interface items in Interface Builder. Cocoa allows you to almost transparently link these items together. Through the magic of the Cocoa framework, they all become connected at runtime.
Also, make sure to learn to use the debugger in Project Builder. It may be confusing and overwhelm you at first, but, trust me, if you learn a few commands (that is, 'po,' also known as print object), you will be able to understand more about what is happening in your application. Set breakpoints on all your functions, especially delegate methods, to see when they are called and what parameters are being passed. Type 'help' in the debugger for more information.
With a little commitment and some hard work, it will all start to come together for you and you will be poised to build the next killer app!