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?
Most often, I would repeat 这是, so if you would ask me 这是什么? I would say 这是一本书. You can also say 一本书. But you can also just say 书.
I am a native speaker, so I can tell you that the most natural answer is "书". Of course, as you know, both are right. But in native Chinese, if you answer the question of "what", no matter only one object or more, you don't need to express numerals.
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.
Just 书 is enough. This is the most natural to answer such a question, and no need to say more.