I don't remember much about this book except that I've read this maybe five years ago. It's a young adult book about a society surrounded by this big wall and the two main classes of people are the poor and the rich. The main characters are twins (brother and sister) and the sister was taken away for some government experiment. A part of the plot was the brother wondering what was beyond this wall and his sister helps him see the truth that it is this beautiful open landscape with little pollution. They end up talking to a high ranking government official about why they were all walled in this city and he explains that the rich people were sick of everyone polluting the planet so they put everyone in this walled city and kept a good chunk of the land to themselves. He then explains that the only reason he got a high position in government was because he knew what about the plot, and now they are planning to revolt or something like that.

Edit: more details

Sci-Fi, it was hardback and it was pretty long (~300 pages? Just a guess)

I read it sometime between 2010-2014