Throughout the book the authors demonstrate how to build a somewhat useful web app. Overall, I think the authors strike the right balance between being trivial and being too advanced. However, their narration style makes the book somewhat difficult to follow, because:
1) There are many references to specific git commits in the book, which suggests the readers look at git repo while reading the book. This is not convenient on mobile devices.
2) There are virtually no pictures in this book (sans screenshots). EmberJS framework’s complexity is far from trivial, so a class diagram would make it much easier to understand the relationship between models, routes, views, etc.
In an advanced book like this, screenshots are mostly useless - your readers already know how to find Firefox extension. A URL will be enough.
Also, it would be good to see how EmberJS compares to other popular frameworks (AngularJS, Backbone), because each of them has their pros and cons. Many readers (including myself) have some experience with some of them. Without such comparison, EmberJS looked to me to be “just another AngularJS”, until I reached the end of the book and found out more about persistent data framework, which appears to be superior of AngularJS’s resource framework. So, this chapter should be moved to the front of the book, otherwise there is little chance that readers will be interested enough to read to the end of book, before they find out something new.