I think 'The Dark Knight Returns' is a must - read. It redefined Batman in many ways and has a very dark and gritty tone. Also, 'The Killing Joke' is essential as it gives a lot of insight into the Joker's character.
Well, 'Long Halloween' is a must - read. It shows Batman's early detective work and his relationship with the villains in a really interesting way. 'Hush' is also very good. It has a lot of action and mystery, and it brings together many of Batman's well - known foes in an engaging plot.