The always-revelatory Inside the Cosmic Cube is looking into the alchemical origins of the Joker. I'd just like to add that the Joker's original incarnation as the Red Hood reminds me of the Rubeo from Alchemical iconography...
Also, it seems that Heath Ledger had read The Killing Joke
DRE: Have you read many Batman comics?
HL: No and I think that’s kind of helping me a little bit. I was never really a fan of comic books or comic book movies. I never despised them but I was never one to read them. I never sought out the films but I would sit down and enjoy them. So because of that I really feel that I’m not carrying much pressure.
DRE: Have they given or asked you to read certain comics?
HL: The Killing Joke was the one that was handed to me. I think it’s going to be the beginning of The Joker. I guess that book explains a little bit of where he’s from but not too much. From what I’ve gathered, there isn’t a lot of information about The Joker and it’s left that way.