One of the most difficult aspects to reviewing books is considering what the author is trying to do, or, more importantly, what the readers will expect from this book.
I just finished Bringing Up the Bodies by Hilary Mantel. Before writing the review, I went back to read my review of the first book in her trilogy, Wolf Hall. In that review I stated that I like to test historical fiction by imagining the names of the historical figures being replaced with other names to see if the book works on its own. I'm not sure that's fair. When the subject is someone famous readers bring knowledge to the process and a good writer should consider that knowledge.
It's all right to make things up. Hilary Mantel's books are mostly dialogue. She can't know what was actually said, so the real test is how believable the story is. Some things can't be changed. For example, I read a book once where Napoleon flew to a battle site on the back of a dragon. I can accept that in a fantasy. But if you tell me Napoleon was a tall man without some elaborate explanations, I'm going to stop reading. It's too much a part of what I know about the historical figure.
I think I was wrong in that criticism of Wolf Hall and I think realizing it made my review of Bringing Up the Bodies better.