The Admissions Committee at Columbia University wants to get to know you more personally. This desire might lead them to ask questions about what you do in your personal time. By asking what you have recently read (or are currently reading), the decision-makers can learn more about your interests and if you place importance on feeding your mind regularly.
Be prepared to discuss a book or publication that you have read or you are currently reading and why you chose to immerse yourself in that topic. If possible, connect this resource to your career goals or personal aspirations.