I really like the idea in the post! For some time already I feel that passing data between the controller and the view by setting an instance variable is one of the biggest and most fundamental design flaws of Rails – and I think you provide great examples of how to avoid it.

However, personally I'm not so sure about this keeping the controller action as a one-line render-only thing. Controllers are procedural in nature and I actually like to see what is happening in a step-by-step way. I can understand the allure of hiding the complexity of filtering books in a private method, but honestly I would prefer to at least set the user directly:

def index
  user = User.find(params[:user_id])
  books = get_filtered_books
  render locals: { user:, books: }
end

I wonder what is your take on that.