Insurgent is the second book in the Divergent series. I highly suggest anyone interested in this book to read Divergent first. This book would struggle as a stand alone in my opinion. Veronica Roth continues the story of Tris and her battles caused by her being Divergent (which means she fits into more than one faction.)
Tris, Four, and the rest of the group flee to the Amity farms outside of the city in order to hide from Erudite/Dauntless. Tris is understandably troubled by the things she done, and the loss of both of her parents. She had become more aggressive. Their respite doesn't last long at Amity and they catch a train back to the city.
It's hard to tell anyone much more past this point without spoiling anything. Almost every chapter reveals something new to the reader. Tris is put through some really terrible ordeals, and I could feel her struggle. Veronica Roth does a great job of putting the reader into Tris' shoes.
Tris and Four find solace in each other, but they are both keeping secrets. When some of the secrets come out, Four attacks Tris for not being honest and upfront with him. I became very frustrated with him for that because he had been doing the exact same thing to her... hypocrite.
In comparison to the movie, there are a lot of differences. They follow the same general sequence, but some of the specifics or reasoning is changed. I don't know how to explain any better without spoilers. I will say that I preferred the movie's version better than the book, but I did watch it before reading the book so I'm probably biased. To me it just made more sense.
Overall, I really liked the book. It was nowhere near as good as Divergent, but it was in no way a disappointment. I do find myself torn as to if I should read the last book before the movie comes out, or continue as I have and read the book after I see the movie. I know most readers will always say to read the book first, but then I will most likely be disappointed in the movie. Personal struggles, anyways, the book was great and I highly recommend to most audiences.