Firsts guys, thank you for putting this together, I know writing books is not an easy process.
I’m started reading the Beta. In page 5, section Your First Spec, IMHO maybe will be best no to have the line
Sandwich = Struct.new(:taste, :toppings) in 01-getting-started/02/spec/sandwich_spec.rb because that line as you know implements the spec that you are defining. I know in a book you need to find a balance about what to cover and what not, and also to serve different audiences, but this seems a section for newbies, then I think to expose them briefly from the beginning to the Red, Green, Refactor cycle will be useful.
Please consider this, present first the Spec thinking about the kind of code we will like to have, and then after failing in Red to add the easiest implementation, in this case, the Struct.