It would be interesting to question those children to find out what they had in mind with their question. This can be done by getting them to guess a couple of possible answers so that you get a better idea of what they are thinking about in asking the question. However, they may not be about the nature of language, and how objects get their names. In any case, a good etymological dictionary can be helpful to show children of this age how language evolved and how people have studied and recorded the development of our language, which itself is made up of words which came from other languages. Some of the questions are very basic and practical.
Books are physical objects which can be handled and analyzed in terms of their parts.
I suspect that children who had undertaken project work since their kindergarten years would not be asking such questions at grade five. They would already have drawn part-whole pictures of books and labeled the parts. Once the mechanics of book-making are dealt with, the child may want to know more about the purpose of books and what goes into them. These are about the authoring and design of books and what has to go into them. Here, on the one hand, there is basic knowledge, like what is the minimum required of an object before it can be called a book.