One thing I've found really lacking in most Rails books is how to properly do JS. I think you should just gloss over the JS helpers and focus how to properly do JS inside a rails app. Fundamental things like using JSON for separation between controllers and front end. What to use wants.js for. And how doing your JS horribly can really muck up your app.
JS Testing too. Integrating with cucumber/capybara. Not sure if this outside the scope or not :)