Batman: The Killing Joke.

This graphic novel Is the best depiction of the Joker's backstory and his very twisted relationship with the Dark Knight! I've somehow recently gotten a hold of a copy that was originally sold within the same publication year of its release. Yet before then I had read said novel in various libraries. Now I won't go into comparisons with the recently released animated movie (mostly because I haven't seen it yet.) but I stand my ground when I say it's the best!