If you're just venturing into comics it's probably best to look at one or two from the heroes that you like.
There's the obligatory "The Dark Knight Returns" of course, and then there's "The Killing Joke" which are the essential Batman reading.
For Kingdom Come it helps to have a little background knowledge of most of DCs heroes because the majority make an appearance even if they don't have a speaking role.
I'll have a look round at some others for you when i get a moment
EDIT: It would appear Jace Falco beat me to it