Illuminae / Amie Kaufman & Jay Kristoff.

One moment, Kady Grant and Ezra Mason have nothing bigger to worry about than each other. Specifically, avoiding each other in the wake of their messy break-up. In the next second, their entire world falls apart. The year is 2575 and one of the mega-corporations that control much of deep space has just fired the opening salvo in an intergalactic war, destroying Kady and Ezra's planet. Forced to flee on a small fleet of crippled rescue ships alongside thousands of other refugees, the fear of enemy warships chasing them down is at first all-consuming but soon becomes the least of their worries. A deadly plague is ravaging the refugees on the ships; the fleet's AI, which should be protecting them, may actually be an enemy; and High Command is refusing to acknowledge that there may be a serious problem. As Kady plunges into a tangled web of data in search of the truth, she realises that Ezra is possibly the only person who can help her save the refugees before it's too late.