As many of you know, I am a fan of Backbone.js. In fact, it has been the subject of talks that I have given recently. After the talks, people often ask for more references on backbone, but other than a few websites, there wasn't much available. But now there is and it is free!
Backbone Fundamentals is a book by Googler and backbone expert, Addy Osmani. The book is available in epub format for free on GitHub. Here is the link: Backbone Fundamentals.
The contains 165 pages divided into 10 chapters. It covers a lot of topics, things like the fundamentals, the internal workings of Backbone, building an application, common problems and solutions. It even includes one of my favorite subjects, building mobile applications with Backbone and jQuery Mobile. It has an easy to read, casually informative style which is packed fully of coding knowledge. And it can't be beat for the price, so be sure to download it.