The author describes how to test Backbone models, views, and collections; dealing with asynchrony; provides useful testing heuristics, including isolating components to reduce dependencies; when to use stubs and mocks and fake servers; and test automation with PhantomJS. He does not, however, teach you Backbone.js itself; for that, you'll need another book.
There are a few areas which I thought were dealt with too lightly. There's no real discussion of Test-driven_development or Behavior-driven_development, which provide the intellectual foundations of much of the book. Nor does he have much to say about testability and how to make legacy code more testable. The sample Notes app has plenty of testing seams (much of this falls naturally out of the architecture of Backbone); other people's apps are not so lucky. The chapter on automation is extremely terse—it could be expanded into a very large book!—but it does provide useful indicators to many areas for exploration.
I learned a lot from this book and I have no hesitation in recommending it.
Disclosure: Thanks to Ryan Roemer and Packt for a review copy of this book.