If you were to actually find Abe's journal from the 1800s, it would be hard to read: he would constantly be misspelling things at first (since he didn't go to school); his syntax would be weird; he would use words that he knows but that we don't; and he would make reference to historical things that we don't understand.
It's lucky for us, then, that Seth Grahame-Smith is around to make this story a lot more accessible. How does he do that? By putting himself in the frame story in the introduction. Since he's a 21st-century dude, he can provide all kinds of footnotes and explanations for us. Seth explains Abe and makes him easier to understand.
For instance, the narrator often stops the story to rewind and give us the big picture. Check out Chapter 10. We start with a mysterious figure being ushered onto a train. Who is it? Well, we don't have to wait long, because Grahame-Smith toggles on back to tell us that this mysterious figure is Lincoln and then goes on to tell us how Abe got to this point in the story. Grahame-Smith may sometimes start with a "mysterious figure" type of mystery, but he always wants the reader to catch up quickly.