Shield of Sparrows

Princess Odessa Cross, the royal nobody expected anything from, is chosen as a treaty bride by a foreign prince who invoked an ancient rite to pick her over her sister. Arranged marriage, a grumpy Guardian escort, and a kingdom full of monsters.

The Crux migration has finally arrived. Odessa is separated from Ransom, kidnapped by a priest while protecting a child, and has to work out what being the Guardian means. It went to number one on the New York Times list in April.