I've gone out on a limb here, and have to say that if just want to read a comic about what the Dark Knight is really about, read Year One!
Better still, read Year One, and then DKR immediately afterwards, you'll witness Batman in his Begins (origin) phase, and then later, seeing a futuristic but still crime infected Gotham City, after Bruce Wayne's retirement, when indeed, the Dark Knight is compelled to return.
Year One is simply told, but Miller and Mazzuchelli tell it so well. There are no villains and no Robin, but it only serves to make the story more gritty and fat free, solely concentrating on Batman's early days starting out.
But it's the details that really stand out, and not just details in the artwork. Miller's story has an earthy realistic edge to it, giving space for characters like Jim Gordon to breathe, even seeing him having a fling, and never making a man dressed as a Bat unbelievable. Chris Nolan would later adopt the same realistic setting for Begins and The Dark Knight. It's groundbreaking in it's approach, and thoroughly deserves the reputation that it has gathered.