In English, if someone points to a book and says "What's that?", "A book" is a sufficient reply. In Chinese, if someone points to a book and says "这是什么？", can you say "书" or maybe "一本书"? What's the most natural and conversational response?
I'm a native speaker. I will just say "书" in most cases. However, the way to answer depends on the situation. It's OK to say "书" between friends, relatives, and colleagues, etc. But when the question is from a senior or respected person, simple answer like "书" could make him/her feel that you are indifferent. In these situation, we will usually elaborate the answer a little bit, such as adding a very brief description about what sort of book it is.