Book 3. Serious spoilers ahead.
This is the capstone of the trilogy. Unlike Gemina, which did not need Illuminae absolutely, this needs both of the first.
As in, the refugees from the first two books are heading back to where it started -- their only chance -- and meanwhile, their being some survivors of the attack on the planet, because BeiTech needs them to mine. One is Kady's cousin Asha, who learns that her ex-boyfriend Rhys is in the BeiTech forces.
Less focus on the central couple that in the other books, because other plot threads that get interwoven. Also, I think it gets more grim than the first one.
It involves second chances, a clever little mouse, passing notes, a parachute, a mass funeral and a mutiny, using a bulletin board and codes, several card games, short rations, and more.