This is how you lose the time war / Amal El-Mohtar & Max Gladstone.

Two time-travelling agents from warring futures, working their way through the past, begin to exchange letters - and fall in love. 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.