This Is How You Lose The Time War by Amal El-Mohtar and Max Gladstone.

"Red, an operative of The Agency and Blue, an operative of Garden are able to travel through time and space, making corrections, deletions, all manner of alterations that will affect time in past, present and future. When Red stumbles upon a letter in the aftermath of a battle that says, "Burn before Reading" she rises to the bait and that's when she "meets" Blue and their correspondence will be the thing that changes everything." 

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Pros: It was definitely an interesting read, told mostly through correspondence between two enemy agents in the Time War/Red and Blue's differences in beliefs first made them enemies but as time progressed and their letters, first taunts and jeers changed to friendship and love, they too, changed as people. 

Cons: The times between the letters, when Red and Blue complete assignments and try to keep their correspondence a secret, could be a little confusing/Since they could travel back and forth through different "strands" of time, I eventually gave up trying to figure out where or when they were. 

Full Review: 

Red is part of the Agency. A technotopia that is ruled by Commandant. They're raised in vats, carefully monitored, enhanced as weapons and sent off into the Time War. 

Blue is part of Garden. An organic, living, growing community of agents that are raised from the Earth, enhanced as weapons and sent off into the Time War. 

Red and Blue first meet in a distant star battle in some vast strand of time. And when Red sees a letter in the aftermath of the grand battle that says "Burn after reading" she finds a letter from Blue, her enemy, basically taunting her and inviting her to give chase. 

Red decides to follow Blue's jeers and see where they lead. And of course, their correspondence through vast reaches of time and space grows from somewhat snarky and condescending into an exploration of themselves, curiosity about their different worlds and soon, they form a friendship. And even more, they realize that they love one another. 

It's betrayal of the highest sort and they both know that if they're caught, it will mean certain death. So, they do what they can to protect one another, even if it costs them their lives. 

This was definitely an interesting, engrossing and quick read. I liked how Red and Blue's letters were at first slightly mocking, jeering, a challenge. But as they learn more about one another, they realize that they care for each other and as time progresses, they also realize that, this war isn't what matters. Red and Blue, one to the other, is what really matters. It was a story about loyalty, beliefs, progress, change and of course, love. And it was told mostly through letters that they wrote to one another. This was definitely one of the more unique books I've read in awhile but unique is simply different and in this case, it's a good thing.