Essential Batman.

Batman: Year One, Frank Miller.
The Dark Knight Returns, Frank Miller.
The Killing Joke, Alan Moore.*
The Long Halloween & Dark Victory, Jeph Loeb.
Hush.




*Note it is supposed to be a one off fictional story within the universe (a dream within a dream). It's been incorporated into the mythos, but that was not Moore's intention. Just don't blame him for Oracle.