This Is How You Lose the Time War - By Amal El-Mohtar

Among the ashes of a dying world, an agent of the Commandment 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, becomes something more. Something epic. Something romantic. Something that could change the past and the future.
Except the discovery of their bond would mean the death of each of them. There’s still a war going on, after all. And someone has to win. 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.