This is one of the better books about the fair, in the sense that the author is a professionally trained scholar (maybe not a historian, but not far off) crafting an analysis and paying attention to the broader historical context in which the fair took place. Brown is clearly well versed in the intricacies of photography, too, and those who know little about the technical aspects of the field (like me!) may be a bit out of our depth in some places.
I'm assigning it in my graduate research seminar on the fair.