Insurgent by Veronica Roth
Tris, formerly known as Beatrice, and her boyfriend Four are at the heart of the second book in the Divergent series. They join forces with members of Abegnation and later some of their Dauntless family in order to battle the mind-control invasion of Erudite.
A war has begun within the walls. Other than the woman who orchestrated the whole thing, only one man is certain why it happened, everyone else is speculating. That one man has proven himself to be cruel and untrustworthy. Four will absolutely not listen but Tris thinks that despite his flaws, he may just be telling the truth. In the meantime, they must try to stay alive and save as many others as they can.
While they stay with the Amity group outside the walls, Tris realizes why she could never be part of their faction. Their happy peacefulness is so contrary to her nature that she must leave before she explodes. When they seek refuge with Cantor, their black and white way of seeing the world is both refreshing and painful. Revelations of truth cause a strain in the relationship between Tris and Four. Being young in a young relationship, they aren't quite sure how to handle absolute honesty.
Four decides to ally himself with his estranged mother and her group of Factionless. Tris is increasingly weary of her agenda and finds herself an ally with Four's cruel father in attempt to reveal what the leader of Erudite started a war to keep hidden. As Four and Tris grow more familiar with the attributes and flaws of each faction, they become more certain that a world of factions is no way to live. The way they each seek a new world could tear them apart forever or the truth of it all could bind them like nothing else could.