Divine Rivals by Rebecca Ross

To combat her worries, Iris writes letters to her brother and slips them beneath her wardrobe door, where they vanish—into the hands of Roman Kitt, her cold and handsome rival at the paper. When he anonymously writes Iris back, the two of them forge a connection that will follow Iris all the way to the front lines of battle: for her brother, the fate of mankind, and love. Shadow and Bone meets Lore in Rebecca Ross's Divine Rivals, an epic enemies-to-lovers fantasy novel filled with hope and heartbreak, and the unparalleled power of love.