The Dark Of Twilight by Kate Danley
Half a kingdom turns into wolves at night and half during the day, but one girl does not shift and she must save them all.
Aein wanted to be a soldier in Lord Arnkell’s army since she was a child. When that day finally comes and she is sent to patrol the border’s swamp with fellow warrior, Lars, it seems like her dream has come true.
But something follows her home.
When Lord Arnkell’s wedding is interrupted by a werewolf attack, the dream shifts into a nightmare… and Aein is the only one awake.