Your interviewers are primarily interested in discerning what your reading material says about you. Not only are they interested to know if you feed your mind regularly, but they're curious about what kind of literature you prefer and why. What you volunteer stands to offer them insights into your personality, emotional maturity, professionalism, and intellectualism. If you're reading multiple things simultaneously, share them all, whether they be books (fiction, nonfiction, literature), a paper, a study, a thesis, or anything. However, whatever you volunteer, make sure most of the materials you offer are of a collegiate reading level or higher. If you are not currently reading anything because you're devoting your attention to other priorities, that's fine, as long as you share something you've recently read that impacted you. When this is the case, it couldn't hurt to suggest you have a stockpile of materials you are looking forward to reading next.