This. I honestly don't like TDK all that much (repeat viewings have soured it for me utterly, I find it way too dark and cruel, personally), but there was nothing about Ledger's Joker that really correlated with the versions that have appeared in the books, especially with them saying they'd looked at everything from his original appearance to the current stuff.
To me, Ledger's Joker was more akin to Charles Manson than anything else. And in fact, the Joker's plot in TDK has an awful lot in common with Manson's 'Helter Skelter' ideas.