It's book 3 in Alex Aster's Lightlark series, Skyshade. Picking up directly after the massive war on Lightlark that was supposed to (in Isla's eyes) cement Grim and all of Nightshade as the evil beings they are, she's instead learned that before her memories were taken she actually died in battle, Grim saved her life, and they fell in love and got married -- whoops! So she's going to feel out that whole situation by returning with Grim to be his wife, but as you and Laura and Meggie know, there's sure to be another obstacle for our fateful protagonist just around the corner.