I think that Batgirl issues #1-6 that will be seen in tradepaperback form this upcoming July in Batgirl: The Darkest Reflection should be considered part of the same tale of The Killing Joke. Part one/chapter one was about the Joker trying to prove that life is essentially meaningless and part two/chapter two is about his primary victim Barbara who overcomes what happened to her and going right back out there as Batgirl to help others, thus completely invalidating what the Joker was trying to prove in TKJ, far better than Batman did, IMO over the course of these issues simply by being a survivor.
I see The Killing Joke as an incomplete chapter that now requires the reading of Batgirl #1-6 to get the full story of how the Joker's nihilistic philosphy is completely disproven by Batgirl, who accomplishes this by simply living her life, finding meaning and moving on and eventually becoming a crimefighter again on the rooftops and in the sewers of Gotham. Batgirl philisophically has proven herself to be the antithesis of what the Joker stands for, far more than the standard hero/archenemy dynamic between Bruce and the mad comedian. This show an INCREDIBLY far more inspirational tale if TKJ and Batgirl: TDR are bookended up against each other IMO, and an inspirational tale for many new readers who take both parts as just one tale that tells a greater whole.