You may have to do some fact checking

If she was 12 years old in 1860, the author might choose to make her 30 years old instead.It is always helpful for readers to know what facts the author has manipulated so that they are clear on fact versus fiction. Many authors will let you know where they have taken liberties for the sake of telling a good story versus whether or not they have stayed as close to the facts as possible.

But that, too, is up to the author;if you want to be sure, you may have to do some fact checking on your own.It's important to do your own research either before or after you read the book—or at least read what the author has to say about his recounting of events—before drawing any conclusions as to its historical veracity.