Oberdorfer's The Two Koreas is excellent, he covers the history of both the North and the South (hard to explain the history of one without the other over the past 60ishbyears). Martin's Under the Loving Care of the Fatherly Leader is all about the North, but also quite good. The Cleanest Race by Brian Myers is more a sociology book than a history text, but gives some good insights into how the North sees itself.

If you're looking to get a better sense of what the country is like for non-elites, Barbara Demick's Nothing to Envy is a good read (again, not really a history book....but still worth it). Finally, if you're looking for a lighter read, the Inspector O novels (starting with A Corpse on the Koryo) are fun, easy to get through, and--allegedly--more or less reflective of the realities of day to day life in Pyongyang.

But for a straight up history, I'd stick with Oberdorfer.