u/Buddhapanda75

So, I'm sure I read The Killing Joke many years ago as one of a handful of graphic novels I had read in my life until last year. I started reading DC comics avidly this year, and I'm about to read A Death in the Family for the first time, but I realized that The Killing Joke happens first. That made me wonder how that works for canon purposes because wasn't The Killing Joke a one-shot? And, furthermore, I understand that Batgirl becomes Oracle... and then Batgirl again. So, can someone explain how The Killing Joke wasn't canon, then was, then wasn't, and now is? or something like that....
I know the plot details of the books and some of the major events from the 90s and up, so you don't have to worry about spoilers in your responses. My question is more about what books are considered elsewhere and how some events become canon.

Hello everyone!
I recently picked up issue 1 of the Swamp Thing 1989, in which he meets Jesus. I understand that this is the conclusion of a larger arc in which Swamp Thing gets unstuck from time. What issue does that specific arc start? I've heard some sources say it's issue 80, and others say it starts at issue 82.
I'm very new to Swamp Thing. I've read his origin in the 1971 House of Secrets (though I think that's a different guy) and the recent Black Label Green Hell. I'll eventually get around to reading more (especially, Moore), but right now I just wanted tog et the full time travelling arc.
Thanks!
Edit: Oh, I also heard one source said that Swamp Thing Annual #5 was part of this arc, as well. Not sure if that's accurate.