Green Rider by Kristen Britain

This series is really good if you are a fan of Harry Potter. It has loads of magic, Intrigue and action (and horses). When Karigan runs away from her school she runs into a man with to arrows in his back (he is dying) he makes her swear to deliver his message to the king. When Karigan has done this she is taken on a 'Wild Ride' all over the country pursued by cutthroats and mercenaries. I really enjoyed the first three and have yet to read the fourth one! (They are by Kristen Britain)
Green Rider
First Rider's Call
The High Kings Tomb
Blackveil