I think we are getting closer in understanding what each of us is talking about. Similar to this forum, I have been in contact with Ian Dees who is co-authoring the Using JRuby book. As I was going through the book, I found a few problems (after all, it is in beta :)), but more importantly, he and I engaged in an active dialog in what CONTENT could improve the book and I am delighted to see that he has followed up on it.
So, what I am saying here is that Rails should not be documented in a vacuum. It has to reflect what works and what has changed. For example, if you see the threads that I mention above, you will see that there are no specific resolutions to the deviations from the standard documentation that is provided by Edge Guides or yourself. So I am merely suggesting that you try out nested layouts and you will see that it contradicts what you are writing in the book. There is always a chance that I am wrong and I would humbly accept it when and if I am proven wrong.
Regarding my comment above “The third edition was the best in the sense that it combined basic rails skills with deeper discussions of intermediate/advanced topics.” It is entirely possible that it is a function of my learning curve too. I was intensely engaged in working on two Rails projects last year and may have found more use of the Third Edition since it was directly applicable to what I was doing then. So, that is a possibility.
I thought that you wanted to keep the more experienced developers interested, so that is where I was coming from. The shopping cart example is very good for beginners and you should keep it there. Go a bit beyond what the Edge Guides are telling us in your in-depth coverage of Rails that is based on what currently works and what does not work and what are the work arounds. For example, the behavior of content_for and yield has changed in some subtle but important ways which I found by listening to Ryan Bates and googling around. That kind of information in a book format is invaluable.. That is what I was leading up to. As you yourself point out above “Furthermore, there is enough differences between Rails 2 and Rails 3 that even people who thoroughly know Rails 2 will need a bit of re-education to once again know what the right questions are to ask.” you are right on target and I have first hand experience of that fact in trying to upgrade my project to Rails 3.