This is How You Lose the Time War by Amal El-Mohtar and Max Gladstone

In the aftermath of a devastating battle, a cybernetic time-soldier finds a letter left by her opponent, addressing her as "Red" and signed with the moniker "Blue." It's a gloating message of victory, but one that sets off a correspondence through time and space. Red and Blue hide words for each other in bones, in seeds, in water, in trees. Chasing each other through alternate histories, they encode these messages in secret, lest their overlord commanders find out and call it treason. Although the messages start out as taunts and boasts, they soon become curious, and then friendly. The stakes rise as their feelings deepen. And over time (pun intended), friendship blossoms into love.

This is How You Lose the Time War is a short sci-fi novel (or novella), mostly consisting of letters between the nemeses and star-crossed lovers Red and Blue as they battle and trick each other through time and space. The letters are entertaining and poetic, although the plot and worldbuilding are twisty and sometimes cryptic. The audiobook is performed by two narrators who both put a lot of emotion into their delivery. It's a quick read if you are looking for a creative and highly unconventional romance.