Some common ways to ask about readings someone did are:
What did you think was the most interesting part of the book?
What did you find interesting (and/or unenteresting) in the book?
When did you feel you had become fully involved or did you decide to give up?
Which is the most attractive or exciting part of the book in your opinion?
How did you like this book/novel?
How did you enjoy this book/novel?
What part of the book did you enjoy most? Why?
What did you enjoy most about the book?
What was the story about?
What does the main character in the book want? What does the main character do?
Who are the main character(s)?
Who is your favourite character?
What is the main idea of the story?
What is the author’s purpose in writing this novel?
Is there one quote from the book/paragraph that stands out to you?
As you see the more open-ended questions the more this invites the reader to express himself, give opinions and make comments.