What is the main point of my book?

After the Civil War there was an event that changed history forever. Abraham Lincoln was shot by John Wilkes Booth in Ford's Theatre. So the main point of my book isn't about the assassination but about catching the killer. The book is interesting because it's a non-fiction book about how they caught Mr. Booth but the way it is told makes it sound like a historical fiction. For my historical fiction I plan to talk about a view point of the chase. The view point will be from a made up character, but have real people from the chase. The conflict will most likely be the group has trouble finding and catching Booth before he's gone for good.