Allegiant, by Veronica Roth+
This book is in both Tris and Four’s points of view. Tris and a couple others escape to outside the wall. What they find is amazing. Her city is an experiment made by the government because of genetic experiments. They would take out the gene that makes people lie but it makes them ruthlessly truthful. Things like that, hence the faction system. Four joins in a rebellion that wants to overthrow the experiments. He doesn’t get all the information though and lots of people are killed and hurt. Tris and Four’s relationship is still very rocky.
Bottom line: This book is a lot better than the last. It does end the series. The ending is very controversial. I think it needed to happen.