As is my custom, I'll be putting my review of Pandemonium behind a jump so as to not spoil the ending of Delirium for those of you who haven't read it yet.
Delirium ended on something of a cliffhanger: Alex had rescued Lena from her family's home, but was shot in the chest by government agents as they fled. Lena escaped into the woods.
Pandemonium takes place a few months to a year after the escape, with the narrative shifting between when Lena first encountered the resistance group about a week after she escaped and the present day, in which Lena and her friends in the resistance are executing a plot in New York City. The government, no longer able to hide the existence of the Invalids and the Resistance, has been cracking down on the population. Meanwhile, the Resistance, with the aid of sympathizers, grows ever-stronger. I can't discuss too much of the plot without spoiling the entire thing, so I'll leave it where it is.
Overall, I liked Pandemonium. Middle novels in trilogies are often somewhat slow and plodding, but Oliver did a nice job keeping it interesting, mainly through the use of the shifting narrative. Had the story been totally linear, it would have become pretty bogged down in the middle. For the most part, she was able to maintain the things that I liked about Delirium: the interesting world, the concept of love as a disease, and the excellent writing and characterization. My only complaint is that the ending and the plot twists leading up to it were pretty obvious and predictable-- though I'm not sure how much of that is the book being formulaic and how much of it is the amount of time I spend reading these novels.
Either way, though, it was really enjoyable. 4 out of 5 stars.