Allegiant by Veronica Roth (2013)
Divergent and continued with Insurgent, so beware that there are spoilers here for those books.
At the end of Insurgent, we learned that the factions were a result of the world becoming unmanageable and violent. The city was created with these factions to restore order and cultivate particular values in its people. When enough people began showing strengths in more than one area - people who were called Divergent, like Tris - it was time to leave the city and help save the outsiders who were presumably still killing each other.
When Allegiant begins, Tris and some others do leave the city, but when they arrive at a place called the Bureau of Genetic Welfare they learn truths that are even more shocking than what they had learned previously. Reeling from this new information, Tris and Tobias must make extremely difficult choices.
The first two books in this series were narrated in first person by Tris, but in this final volume the story alternates between Tris and Tobias. This is a pretty typical mechanism, but unfortunately their voices were almost identical. That's not terribly unusual either but it does make things a bit confusing now and then.
Mostly I liked how the story progressed and thought it was a good ending to the series, even if certain aspects of the resolution seemed a bit too easy. There were some surprises at the end, and I have to admit that I like the choices Roth made. As with the other books in the trilogy, the interpersonal relationships were complicated and nuanced. It wasn't exactly smooth sailing for Tris and Tobias, but Tris's relationship with her brother was even more interesting and some very tough choices had to be made near the end. There was an epilogue that took place a couple of years in the future but, thank goodness, didn't go the Harry Potter or Mockingjay route. It gave you enough about how things turned out, without leaving you feeling strange and unfulfilled. All in all, I found a lot to like in this series and was satisfied with its conclusion.