'This is a book' is a statement which can really only be used when there is a book in front of you and you're indicating the specific book. However, 'it is a book' can be used in different contexts. The book can be referred to whether it's in immediate sight or not.
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Voytaszek2. This is a book.Introducing the concept of a book to people who don't know what a book is.
Voytaszek1. It is a book.Do know what object is in this package? It is a book.
Voytaszek1. It is a book.2. This is a book.You may find this post useful.