The 3rd chapter is now available on the webOS book. The first chapter was free, the rest you got to pay. First chapter of the book talks about the Overview of webOS, the second on the Application Basics, and the third is all about widgets.
Widgets are dynamic UI controls, that can be integrated within any application. They can be tailored to the application, yet provide reusable, stylistically consistent UI functions. Widgets is a term widely used within web development but Mojo widgets are different than other widgets. Mojo widgets have a defined behavior but with many options; they generate complex HTML and are easily styled with CSS.
It helps to understand the HTML that the widget generates. This is especially true for widgets like List and Dialog, where the app specifies HTML templates that largely define the widget's structure.