Naga General’s Mate by Celia Kyle

Dahlia Skysinger has managed to survive the underworld of Astera her whole life through clever deals, selling her voice, and never staying in one place too long. When her little brother is caught stealing from the crown, Lia is forced to make a cruel bargain with the royals.
Her life for his.
To regain her freedom, Lia must impersonate the princess, marry the monstrous frost giant king, then spy on and . . . kill him.
Taking a human bride is King Neve’s last chance to secure peace for his people. Promised a frail mortal princess he could lock away and forget about, the last thing he expects is a woman with a sharp tongue and a reckless amount of courage. Lia might be the key to avoiding war, but he doesn’t trust a single word that falls from her pretty lips.
And so the wicked game of trickery and survival begins.
But what happens when an assassin starts to have feelings for her target, and a king’s icy heart begins to melt for his little human bride?