In the 1980's there was not a better writer in comics than Alan Moore. Back then Alan was capable of fleshing out characters to the point that even the most absurd superhero seemed human (and I assume this is still true today but since Moore's less than amicable split with DC, I havn't read any of his work). The Killing Joke is one of comics best takes on the Joker, delving both into his madness and his sanity. As sadistic and evil as the Joker is, Moore still manages to evoke some sympathy for him.
In The Killing Joke, the Joker has kidnapped Commissioner Gordon, shooting and crippling Barbara Gordon (aka Batgirl) in the process. Batman goes to Arkham Asylum to find out what exactly has happened, as the Joker is still supposed to be locked up in the madhouse, only to find that it's not the Joker there at all. What follows is a deep, thorough and frightening exploration of the Joker's psyche, his history, his relationship with Batman, and his cold, methodical torture of Gordon.
As Frank Miller's Return of the Dark Knight was a turning point in the Batman mythos, creating a deeper, darker Batman, so too is The Killing Joke in the mythos of the Joker, Commissioner Gordon, and Batgirl.
The art by Brian Bolland, with painted colour by John Higgins, is simply amazing. The amount of detail, both in scenery and anatomy, is as real as pen-and-ink will ever get. Bolland is truely the best artist in comics (it's just a shame we don't see more of his work these day). Higgins' colours are effective in setting mood, establishing flashback sequences, and are tell tale signs of the emotions on the page.
The Killing Joke still stands as one of the geatest Batman stories ever.