You want to accomplish two things with the response:
demonstrate you read one of the books
develop some well-thought out questions for the conversation on Wednesday morning. You want to have several questions prepared because you know there will be overlap and because I want you to think more deeply about things than just to come up with one question.
In addition to these two purposes, there is the possibility you could have some comments about the work that you would not want to make in front of the author. You aren’t required to like every book after all. So of course you want your critique to be thoughtful and evidence-based but if you have some things you want to say about the work that is better off not said in front of our guests, the discussion board is the place to do that.