The Seed of Winden. The Red Slipper Series, #5

Nieve spent her entire life in the dry, lifeless lands of the desert, a girl out of place in her own country. Her aunt's recent marriage allows them to move to the one remaining patch of green in the known world-a lush, vibrant oasis that sits upon a plateau, guarded by manpower and magic. This land is everything she hoped for, a place she finally feels at home. Tammin Halenwood was born into power and privilege, a son of one of the greenland's noblest families.
His attraction to Nieve is as strange as it is immediate. But it is their visit to the mysterious stones atop Winden Hill that seals their fates. The act of stepping into the magical circle together sets off a chain of events that endangers the greenlands and exposes the land to the curse of the hungry, living sands. As the desert encroaches into this last vestige of life, destroying everything in its path, it is up to Nieve and Tammin alone to break the curdde and save their world from being consumed by the sand.