Foreshadowing

    This story opens with a letter from the author entitled "Evils Imminent". This one page letter, however short it may be, sets up the tone of the book. Like a warning to the reader, Larson describes how everything in this story is true, using memoirs, letters, and other documents as references. Taking this into account, one can only assume that this story, whether discussing the worlds fair or the dangerous serial killer, will be incredibly factual.
As the letter concludes, Larson sums up what this story will be about, stating that "it is a story of the ineluctable conflict between good and evil, daylight and darkness, the White City and the Black." This use of comparsion helps build the suspense of the story before it even begins. This letter is a good way to bring the reader in, pretty much saying "Hey, with all the craziness going on the book, you might think it's fiction, but surprise! You're wrong." Reading that letter made me curious to find out what was going to happen on the next page, which I'm assuming is what Larson wanted to have happen.
Not only that, but the actual story open with a prologue, discusssing how years after the World's fair, Burnham and his friend Root are both on White Star Line ships. As you may know, the White Star Line created the Olympic and the Titanic, which happened to be the two ships these men were on; Burnham on the Olympic and Root on the Titanic. By mentioning these two ships, one gets an eerie feeling, knowing that the Titanic is going to sink before it can complete its journey, and possibly, Burnham will lose a friend. By mentioning the Titanic, Larson has set up the eerie feeling, as if something wrong is going to unexpectedly happen and completley shake everyone involved to their core. His use of foreshadowing help to create a perfect setting for the story of the World's Fair, given the excitment of the fair a sorrowful, creepy tone that no doubt, will follow throughout the rest of the story.