“Batman: The Killing Joke is a one-shot superhero comic book written by Alan Moore and drawn by Brian Bolland, published by DC Comics in 1988. Dark Knight director Chris Nolan has said that The Killing Joke [served] as an influence for Heath Ledger‘s Joker character. Ledger stated in an interview that he was given a copy of The Killing Joke as reference for the role.” — from the B:TKJ Wikipedia page.