Let me start off by saying what this book is not. It is not a novelization of the movie. It is not what the movie was based on. It is not a tie-in edition.
What it IS is a book about the man who taught the king to speak, written by his grandson and all based on his research in his grandfather's journals, letters, etc.
In fact, he talks in the book about how the people making the movie came to him during production and that's when he started researching.
It is also completely fascinating.