This Is How You Lose the Time War

This Is How You Lose the Time War by Amal El-Mohtar and Max Gladstone, 198 pages.

An epistolary romance that runs up and down the threads of time. (maybe it's not a "romance,"  I dunno, I throw this term around loosely).

Red works for Commandant and the Agency, and Blue, her adversary, is a part of Garden. Garden and the Agency are in an unending war that travels up and down the timestream. 

Both Red and Blue are lone operatives and among the best at what they do. Their adversarial relation begins to change as they begin an exchange of letters, missives, and messages in a variety of formats. They situations with their respective employers grow strained as they become increasingly interested in each other. An odd and enjoyable read. 

Winner of numerous awards.