Alan Moore - The Killing Joke

The Dark Knight has been rarely seen in a darker setting than what Allan Moore has reserved for this brooding tale of desperation and insanity. Set right after the time of the Joker's escape from Arkham Asylum, the absolutely deranged villain seeks to prove exactly what his difference from his ultimate enemy is, and just how small the barrier between reason and pure chaos is.

And his greatest opponent is not Batman in this battle; it is Commissioner Gordon, the man in charge of the Gotham P.D. and the representative of order in the city with the highest crime rate in the world. It is he that the Joker seeks to push over the edge by shooting his adopted daughter - a major event on its own for the D.C. Universe, since Barbara Gordon happened to be Batgirl, and she was crippled by the incident - and then kidnap the Commissioner himself in order to taunt, anger, corrupt, and attempt to show his version of what sanity really is -- it's something as frail as having a single really, really bad day.

The glimpses we get out of the Joker's psyche are invaluable, and ample proof of Alan Moore's mastery of characterization. The quotes used are amazing (his past according to him is "a multiple choice", not a single entity!), his motives so chaotic that it is clearly shown why he is all that Batman faces and his ultimate opponent, and the methods he employs hit the nail on the head when it comes to encapsulating his personality.

As for Batman, he has almost a second role in the story, but by the end it is clearly shown why he is there; he's another person who had a really bad day that changed his life, and the similarities between him and his enemy prove to be numerous.

... And of course, there is the joke at the end. If nothing else, I would recommend reading the graphic novel just for that.