Allegiant is the final book in the Divergent trilogy by Veronica Roth. In this book, Tris & Tobias are offered the chance to leave their city behind and explore life outside the fence. They are hoping to find answers, but instead they find even more questions about the faction-based system that they were used to. They also begin to learn the truth about the history of being "divergent" and what it really means. Tris is used to facing her fears, but she is put to tests she never could've imagined. She has to learn forgiveness, trust and sacrifice in order to live in this new place.
One of the main themes in the series is whether people are born with an inherit skill set genetically that determines their worth, or if choices people make can define them. The ending of this book sure leaves you wondering who Tris is allegiant to and how much being part of the Abnegation faction never left her.
If you've read the first two books, there's no way you can avoid reading this one. It has deep, meaningful themes that make the reader reflect on what humans are worth, and perhaps leaves the reader with more questions than answers.