This Is How You Lose the Time War — Amal El-Mohtar, Max Gladstone

At the heart of the story are two rivals, agents of opposing factions, who are waging a fierce war for the future of humanity as they travel through time and parallel universes:

Red—a representative of “The Agency,” a high-tech, cybernetic future where pure intellect and machines reign supreme.

Blue—a representative of “The Garden,” a biogenic future where everything is based on organic matter, nature, and collective consciousness.

Their mission is to alter the course of history across different timelines (for the benefit of their faction) in order to ultimately win the “War of the Ages.”

It all begins when Red finds a teasing note from Blue on the battlefield, marked “Burn before reading.” Thus begins their secret correspondence. They leave messages for each other in the most unusual places: in tree rings, in a bench, in animal bones, in the flavor of tea, or in the markings of a seal.

Over time, their rivalry grows into a deep understanding, and eventually—into forbidden love. Now their main goal is not to win the war, but to save each other’s lives, because if their superiors find out about this connection, a brutal reckoning awaits them.