"Seth Grahame-Smith" is a name you have to say in quotation marks. We have him in quotation marks because we're not talking about the real dude. We're talking about the character who shows up in the introduction.
This guy works at an old-fashioned store in Rhinebeck, NY (an old-fashioned town), and though he likes books and writing, his regular life has gotten in the way of his dreams: "I stopped caring about my writing, and started caring about everything else: The kids. The marriage. The mortgage. The store" (Introduction.15).
In a way, he reminds us of Abe when Abe is working at a store and gets married to Mary Todd. Working man Abe has much less time to hunt vampires (5.51), and when he gets married, he retires altogether (7.167).
But his dreams fall right onto Seth's doorstep when Henry Sturges reveals to him that vampires are real and gives him Lincoln's secret journals. Maybe Henry chooses this narrator because Henry sees something of Abe in him; as Henry tells Seth, "You obviously have an appreciation for books. I see you writing here from time to time […]" (Introduction.28).
No matter what the motivation, this work takes over Grahame-Smith's life. He becomes obsessed with vampires and his wife even leaves him; but at the end of two years, he has this amazing story that finally tells the truth about Abraham Lincoln, vampire hunter.
And then… that's all we hear about him. What happens after? Does he become a vampire hunter himself? Do we smell a sequel?