A Prestige format book originally published in 1986, The Killing Joke has fast become a favourite primer for Batman comics for several reasons. It's short and sharp, captures the darkness that's now built into Batman on a base level, has consequences, features the Bates undisputed number one villain on top form and has some seriously heavyweight creators. It's a little more Brian Bolland's baby and while he's grizzled about the colour job it's unarguably a brilliant piece of work, with several frames genuine touchstones of Batman history. The face to face between Dark Knight and archenemy (though few remember the latter is actually a dummy in the original) and the concept behind it has been knobbed by everything going, for instance.
Alan Moore has been somewhat disparaging about the book in years since too, though to be honest he is about everything these days anyway, old misery that he is. But it stand up brilliantly and never feels thirty years old. The 'possible' origin tying the Joker into the Golden Age Red Hood has also been widely accepted as canon insofar as anything else DC is canon, precisely because it's so well written, and the interpretation-laden last page still pulls things off - the joke is actually pretty funny, even after many readings, avoiding that whole King of Comedy thing.
In the years since the crippling of Barbara Gordon has copped a lot of flack but this is mainly through 21st century glasses. In the context of a period of comics where status quo was king (Crisis merely establishing a new status quo) the sudden, brutal and casual nature of the Joker just turning up on a hero's doorstep and shooting her through the spine both ramps the danger up and creates a jarring effect. Both the incident and its' aftermath - where Barbara's critically injured body is photographed and possibly abused - serve to show how dangerous the Joker is. It's a deliberately crass attempt to undermine Jim Gordon's belief in civilised society and if it triggers you into repulsion well done, it's meant to. Plus it's a change that gave the huge positive response of the paraplegic Oracle. Meanwhile Jim's complete refusal to be dragged down to the Joker's level despite the callous targeted bad day (compared to the Joker's collision of bad luck and his own extant weakness) is uplifting and makes him on the quiet the real star of the story.
Overall The Killing Joke deserves its exalted status. It's not a perfect comic but it is concise and memorable, a perfect introduction to modern Batman comics in that it leaves somewhere to go while functioning as a compact story based on a dynamic ramped up to legendary status.