Pegasus. Robin McKinley

The first book in a magical two-part tale of fantasy and friendship ...On her twelfth birthday, Princess Sylviianel is ceremonially bound to her own Pegasus, Ebon. For a thousand years humans and pegasi have lived peacefully in the beautiful green country beyond the wild lands. They rely on human magicians and pegasi shamans as their only means of real communication - but not Sylvi and Ebon. Their friendship is like no other...They can understand each other. But as their bond grows more powerful, it becomes dangerous - could their friendship threaten to destroy the peace and safety of their two worlds?