Nonfiction texts can influence the perception of their subject because usually the author has some amount of bias towards that subject. The pieces of information they choose to use and the side of history that they're on might impact the way the subject is seen.
For example: A nonfiction novel about Abraham Lincoln might be written by someone who agreed that his cause was just and right. This might skew some of the way the information about him is presented. Alternatively, if the author thought Lincoln was terrible, they might choose different pieces of his life to highlight, and tell stories in a different way, giving a different perspective.