This is a terrific article on the physics of Dark Energy, and what we can expect in the next decade or so. It was written by two of the leading figures in this field, Eric Linder and Saul Perlmutter.
There's also a rather good article on the history of the discovery of the accelerating universe that led to the idea of dark energy. It was written by Robert Crease. Not only is the story interesting, but it tells you of the workings and the competition between different groups within the same field and trying to study almost the same thing.