Flash the Sheep Dog by Kathleen Fidler
Tom Stokes is an orphan. His sister is going to America to get married, but where can he go? They remember an uncle and aunt they scarcely know who live in the borders of Scotland. After the city bustle of London, Tom finds his uncle's farm barren and lonely. How can he adjust to such a life? Help comes in the form of a sheepdog pup, and his loneliness is forgotten when Tom realizes it is his, to love and to train (maybe to be a champion?). Life on the hill-farm, with his new friend Elspeth, and Flash, becomes adventurous and challenging. Then his sister writes that Tom can come and live with her in America, and he is faced with the most difficult decision of his life.