This Is How You Lose the Time War (Deluxe Edition) by Amal El-Mohtar

Among the ashes of a dying world, an agent of the Commandant finds a letter. It reads: Burn before reading. Thus begins an unlikely correspondence between two rival agents hellbent on securing the best possible future for their warring factions. Now, what began as a taunt, a battlefield boast, grows into something more. Something epic. Something romantic. Something that could change the past and the future. Except the discovery of their bond would mean death for each of them. There's still a war going on, after all. And someone has to win that war. That's how war works. Right? Cowritten by two beloved and award-winning sci-fi writers, This Is How You Lose the Time War is an epic love story spanning time and space.