Right now they lack a lot of documentation similar tonhow rails started at first. An Introductional book like the agile web development with rails would be nice.
Please make a book about sproutcore backed by rails. Taking the depot app and using these two frameworks would be amazing or even a social network app would be fun. If you do another depot app with rails could you include active-merchant as well. Taking this example full circle would be great for the rails community.