Non - fiction. 'Killing Lincoln' details the events leading up to and including the assassination of Lincoln. It's written to inform readers about this significant event in history. It doesn't create fictional characters or events but rather tells the true story as best as possible with the available historical evidence.
The fact that it is based on real events and historical research makes it non - fiction. 'Killing Lincoln' is a retelling of the actual events surrounding Lincoln's assassination. It doesn't deviate from the known historical facts. It includes details about the people like John Wilkes Booth who were directly involved in the act, as well as the political and social context of the time. All these aspects are based on historical evidence, not the imagination of the author, which is why it is a non - fiction book.
It's a matter of religious belief and interpretation. Some see it as based on historical events, while others view it through a more spiritual or symbolic lens. So, it can be both fiction and nonfiction depending on your perspective.