This Is How You Lose the Time War - Amal El-Mohtar
It is impossible to write a decent time-travel novel. Therefore, this book cannot exist.
I bought it purely on the strength of the single-line blurb:
Among the ashes of a dying world, an agent of the Commandant finds a letter. It reads: Burn before reading.
The whole book is poetry. I mean that in the literal sense. There are layers of symbolism and metaphor which I am ill-equipped to decipher. I highly recommend reading a linguist's commentary on the book after you finish it for the first time.
It's tense, exciting, hard sci-fi which is properly unforgiving in its lack of exposition. Heartbreakingly romantic and devastatingly clever. My favourite book this year.