what is this?
can we say 'this is a book'? Yes you can, if the book is close to you. If the book is far from you, you can say 'That is a book.'
why do we say 'it is a book' while replying this kind of questions? Possibly the main reason is that using "It" avoids having to choose between "This" and "That". So "It" is a very common/useful pronoun.