I don't think Seth Graham-Smith would claim that either his novel or the movie was "original". There was an interview with him that I saw where he said the basic genesis of the novel came from seeing vampire books set next to biographies in book stores, on a repeated basis. He got the idea to combine the two from that basically. I think he picked Lincoln because he was fascinated by him. He also expressed a hope that people would, due to the film and book, express an interest in the historical Lincoln.