I'm always reading something, usually multiple books at a time.
Requiem alternates between "Lena" chapters and "Hana" chapters. Although readers have not seen or heard from Hana since Delirium (Delirium, #1), there was a moment in Pandemonium (Delirium, #2) where Lena realized her left-behind friend had reached the date of her scheduled cure. As we catch up with Hana, she prepares for her wedding to her "pair," Fred Hargrove, who is poised to be sworn in as mayor, following the death the of his father, the former mayor. Hana is concerned that her cure might not have worked correctly or completely, as she still has nightmares sometimes. She also has not lost her ability to care, as she worries about what has happened to Lena's family since Lena disappeared on the other side of the border.
Lena's chapters continue where Pandemonium left off, and the stakes for her and her fellow "Invalids" continue to get higher as government forces change their official position from pretending that Invalids do not exist to sending military troops out to decimate them. Lena also struggles to sort out her emotions with the reappearance of Alex, just as her feelings for Julian have deepened. Alex tells her that he never loved her and seems to cozy up to a new arrival, Coral--but things are more complicated than that.
Some thoughts on the ending. From various discussions and reviews, I have seen that many readers were very disappointed that Oliver opted for an open ending. I definitely sympathize. Having read a decent amount of contemporary literary fiction, I have had my share of disappointment over a lack of closure. However, I find that I don't really mind it here. The situation that Oliver depicts is too vast to be wrapped up neatly within the confines of her narrative, with a huge, repressive government to overcome. So she leaves on a hopeful note, with a swelling of resisters and young, no-[yet]-cured people ripping down the border wall. We have a step toward the resolution of the love triangle (which I wasn't so thrilled to see emerge but think Oliver handled better and more realistically than most). We have Lena growing into the role of a fierce protector, inspired by Raven, ready to care for Grace and fight alongside mother Anabel. I might just have to write up some fanfic to wrap up some of these stories, but I don't feel disappointed.
So I highly recommend this entire trilogy and recommend that you have all three books on-hand you that it is not necessary to wait between finishing one and beginning the next. You'll thank me later!