There is a draft of a Ruby standard in the works. You can get it here: http://ruby-std.netlab.jp/
Your example about case and Programming Ruby is poorly chosen. When Programming Ruby introduces case (on page 98 for the first edition; 141 for the second edition), Thomas says,
The Ruby case expression is a powerful beast: a multiway if on steroids. And just to make it even more powerful, it comes in two flavors.
He then briefly explains both ways to use case (with an explicit target after the initial case and without). He actually begins with the style you say he doesn't mention.
Your larger point is not unreasonable though. It will probably be helpful to have a standard.
