Please, could you explain to me the difference between these sentences?

1. It is a book.
2. This is a book.

I`ve never considered this issue despite that I`ve been studying English for almost a year now
'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.

"What is the object on the table?"
"It is a book."

You can't use 'this is a book' in this example as 'this' is a pronoun which indicates something which is 'here'.
By using 'it', you've chosen a pronoun which shows that it's a specific thing but not one which is necessarily visible to us.
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.

Do you know what this is? It is a book.
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Voytaszek1. It is a book.2. This is a book.
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