Though it is not a work of fiction, this book reads a lot like fiction. Primarily, I think, because it contains the things most novels have: plot, setting, characters, point of view, and conflict.
Plot - It is an intricately woven plot, moving between the two primary characters. Their paths run parallel to one another at times, and they intersect at certain moments, as well. There is amazing suspense and drama built into the plot--I mean, one of them is creating a world's fair on a level never before seen, and the other is methodically killing people who have come to be part of that spectacle. You just can't make this stuff up, really.
Setting - I already mentioned the world's fair, a work in progress but with so many intriguing and interesting elements it doesn't seem real to a modern audience. As for Holmes's world, it's a terrifying revelation throughout the book.
Characters - A methodical and maniacal serial killer. Enough said. A relatively unknown man who is both able to gather the greatest minds and talents as well as use his own inventive genius in order to create what the world had never before seen. Plenty of smaller players, but these are two enormously powerful characters.
Point of view - Every novel has one, and this book does, too. It's told in third person by a narrator. This is one element which might have changed if this had been a novel.
Conflict - Well, I think I just laid that out in discussing the elements above. The story is jam-packed with conflict.
Clearly this is a work of non-fiction, because there are elements which bog it down and keep it from reading exactly like a work of fiction; however, it does contain all the classic elements of a novel.