There are many places to learn about Backbone, but perhaps, the best place is Backbone's own source code. As the author of both Backbone and Docco, it's no surprise that Jeremy Ashkenas uses the Docco documentation tool to provide an annotated version of Backbone's source code, which you can find at http://backbonejs.org/docs/backbone.html.

However, you don't need the annotated version to read Backbone's code. In fact, whenever some part of Backbone seems confusing or difficult to understand, one of the best ways to learn more is to throw debugger statements into the Backbone source code itself and then, run your application using the browser's debugging tool. By walking through the code in the debugger, you can see the logic progress through Backbone's classes and methods, and because the source code is so well-written and readable, this task will be much less daunting than it would be with almost any other major JavaScript library.

Backbone also offers a wiki at https://github.com/jashkenas/backbone/wiki. In addition to basic Backbone information, this wiki includes collections of plugins and development tools, lists of companies that use Backbone, and a great variety of tutorials and informative blog posts. This latter list (which can be found at https://github.com/jashkenas/backbone/wiki/Tutorials,-blog-posts-and-example-sites) is particularly valuable, with more than fifty different high-quality sites where you can learn more about Backbone.

Another great source of information on Backbone is Packt Publishing, which offers many great Backbone-focused books. While they do cover some of the same basics as this book, you might find the sample applications built in BackboneJS Blueprints by Andrew Bugess or the recipes of Backbone.js Cookbook by Vadim Mirgood to be valuable. If you'd prefer a more advanced look at useful Backbone patterns you can employ, then you might instead find Backbone.js Patterns and Best Practices by Swarnendu De useful. Further, if you find your interest piqued by Chapter 10, Keeping the Bugs Out: How to Test a Backbone Application, then Backbone.js Testing by Ryan Roemer is a perfect text for you to learn more about Backbone testing.

However, as great as books are, they can never keep completely up-to-date with the very latest in emerging Backbone technologies, and that's where certain websites can be invaluable. One incredible resource is Stack Overflow (http://www.stackoverflow.com), where programmers of any persuasion can find answers to their technical questions, including ones about Backbone. However, Stack Overflow can also be valuable when you don't have a specific question. Because the site has both tags and votes on every question, you can search for questions with the backbone.js tag, and then sort them by votes; any questions at the top will most likely be educational. At the time of writing, Stack Overflow featured more than 12,000 different Backbone questions with answers (and more than 17,000 questions in total).

Another similar question and answer site, which isn't programming-focused, is Quora (https://www.quora.com/). While Stack Overflow limits itself exclusively to objective questions, Quora has no such limitation and so is perfect for more subjective questions, such as "What are the advantages of Backbone.js" (http://www.quora.com/What-are-the-advantages-of-Backbone-js).

Another excellent place to learn about Backbone is the Backbone Google Group (https://groups.google.com/forum/#!forum/backbonejs), which has an active community. If, on the other hand, you prefer to follow a stream of news articles, the Backbone sub-feed on Reddit (http://www.reddit.com/r/backbonejs/) is a great resource for keeping up to date. Finally, on a more general level, Hacker News (https://news.ycombinator.com/), Lobsters (https://lobste.rs), and Dzone (http://www.dzone.com/links/index.html) all feature continuous feeds of various programming-related news and articles, including many on JavaScript in general and Backbone in particular.