Legend

The Legend trilogy features first-person narratives by the main characters, Day and June, who live in what used to be the western United States but is not called the Republic. Day, born in the slums, and June, born into an elite family, are opposites in many ways but both want to know the truth.

The plot of this trilogy revolves around the war between the Republic and the counter-offensives of a rebel group called The Patriots.     

It doesn't get much better than this - plagues, class warfare, maniacal soldiers, and a budding romance between the two main characters that twists and turns so much you don't see the end coming at you.

Lu also wrote a prequel, Life Before Legend, that provides more back story to June, Day, their families, and the Republic.