It starts in a slightly odd manner. There is a chapter discussing the benefits of cross browser development, followed by a chapter in which the author writes a simple testing framework. This framework allows you to embed tests inside an HTML page, and displays the results of the assertions inside the tests in an easy to read manner. Lots of the explanations that follow in the book will be expressed as assertions in the language of this testing framework. This way of working works very well.
The next section, Ninja Training, first talks about runtime code evaluation, at first using eval and Function to make new function objects, but then moving on to describe the inbuilt decompilation (as you can ask a Function for its source code). There is then an interesting section showing some of the uses of these techniques.
There are then two chapters on cross-browser strategies for using the DOM. Lots of practical discussion of the differences between browsers which taught me a lot about the DOM and how it should be used.
The last section, Master Training, also looks at browser differences, looking at the differences in the event model and CSS selectors.