Short version: expect an early beta of about 1/3 of the book to be out in about a week.
Work is proceeding steadily, the current draft of just hit 30 pages toward a target of around 90, though about 10 of that is the same intro and outro that the other books have, so it’s more like 20 pages toward a target of 80.
The structure of Book 3 is one chapter on replacing the existing app homepage with a very simple Backbone clone, then another chapter building a different page using Backbone that has more events and more complexity, then a third chapter talking about communication between Backbone and the server, and anything else that comes up. The exact division between the second and third chapters may change.
I will release the first beta when the first chapter is draft complete — this will be rougher than the other books because it hasn’t gone through rounds of review and editing, but I like how it’s going. I also need to clear through reported errata on the first two books, and probably clean up the landing page in expectation of doing another marketing push.
Thanks for being patient and for your great feedback.
Oh, and you can still buy it!