There have already been a few key chapters in this book. For example, what would a book about an AJAX framework be without a chapter about client/server communication? Or, without a chapter about data-bound widgets, the elegant way to divide data content, and view structure in a widget made possible by AJAX?
Here is another key chapter. Its title could have been: Squaring the Circles of Web Applications, which also could have been the subtitle of this book. It is about building your own UI component with the Tag Definition Language (TDL), where the UI component is encapsulated as an XML tag in a custom namespace to allow it to be placed on a web page as a first class citizen, just like any XHTML or BTL element.
Even the most complete library of UI widgets will never cover all the requirements of a particular application. The standard way of extending what you need is adding some JavaScript here and there, often resulting in a spaghetti bowl of JavaScript functions...