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.
To be exact, he kept seeing books about Abraham Lincoln in display next to the Twilight books. This being 2009, Lincoln's bicentennial and Twilight was really starting to build its popularity thanks to the movies, thus explaining why there were so many books on both subjects at the stores.
This got him thinking about merging Lincoln with vampires somehow. He thought about making Lincoln a vampire, but realized this didn't make sense and so made him a vampire hunter instead.