Siege and Storm (Grisha Trilogy #2) by Leigh Bardugo. 2013. Softcover (YA)

Hunted across the True Sea, haunted by the lives she took on the Fold, Alina must try to make a life with Mal in an unfamiliar land, all while keeping her identity as the Sun Summoner a secret. The Darkling has emerged from the Shadow Fold with a terrifying new power and a dangerous plan that will test the very boundaries of the natural world.