Finnglas and the Stones of Choosing — Fay Sampson

"On, Cloudâ€"Clearer, on!" screamed Princess Finnglas. But she knew her horse was already pouring out all his strength. And the rival black stallion was still ahead. Finnglas is riding for her life and the crown. If she loses, she will die and the Summer Land will fall into the grip of evil powers. But this race is only the first of the Seven Trials. There are more terrifying tests ahead.