Notes from the Burning Age by Claire North
Notes from the Burning Age takes place in a time after the world burned. The kakuy, spirits of earth, sky, water, became so enraged at our destruction of the environment that they burned and drowned us. Those who survived became more careful of how they use resources and honor and fear the kakuy. Some people, like Ven, study ancient texts, learning from them but also labeling violent and earth-damaging knowledge as heretical. But a war is coming, when some would use the secrets of the past for their own gain.
Yes, this is speculative fiction, but at heart it's a spy thriller, a tightly plotted novel with memorable characters and plenty of twists and harrowing situations. The information is being gathered from the past, leading to translation issues, research espionage, and a situation where knowledge is power, and the war rests on who can control the most knowledge and use it most effectively. The relationships in the book are believable...