I believe its just a whole number star rating.
Not really sure why you would want to copy Amazons star rating, unless it's just for reference Books like many things are mostly subjective, so its YOUR rating thats important.
I suppose you could get an idea on the book via Amazons rating, but you could then also miss out on books because you put too much stock in that rating and end up not reading that book.
I find it best to just rate books as you read them, and not worry about others opinions of them.
You could always create a custom column and have your own rating system using numbers I imagine, and copy Amazons rating that way if its of importance to you