The notebook, featuring photos, drawings and writings inspired by the villainous Joker, was made by the late actor before he filmed the hit 2008 Batman blockbuster, for which he won a posthumous Academy Award.
Video footage of the book, from the German documentary series Too Young to Die, has emerged online and shows Ledger's father Kim discussing his son's work as he leafs through the pages.
He says, "In order to inhabit his character, he locked himself up in a hotel room for weeks. He would do that. He liked to dive into his characters, but this time he really took it up a notch."
Kim also admits he was emotional reading the last entry, which shows the haunting words "bye bye" written across the page, adding, "It was hard to see this."
The Australian actor was just 28 years old when he died from an accidental prescription drug overdose in 2008.