Our hero is actually a heroine this time.  The Green Riders are the Royal couriers, or Norse riding messengers.  Our heroine is on her way home from school in disgrace for defending herd of for on a noble-born bully.  A Green Rider, mortally wounded, emerges from the woods and more or less demands that she delivers a sealed message to the King.  Reluctantly, she agrees, and ends up with his horse, his badge of office, and his horse.  Fans of Mercedes Lackey will like this book. By the way, this is the first volume in a six-book series.