Below is a list of books that are rumored to be the answer to this question: What books inspired Heath Ledger's portrayal of The Joker?

1. Arkham Asylum: A Serious Home on Serious Earth

2. The Killing Joke

3. Batman: The Dark Knight

4. Batman: Dark Victory

5. Batman: The Long Halloween

6. Batman: The Man Who Laughs

7. Batman #1 

8. Detective Comics # 40 The Murders of Clayface