Under the Lilacs - Louisa May Alcott

When two young girls decide to have a tea party with their dolls and a mysterious dog comes and eats their prized cake, they end up finding a circus run-away, Ben Brown. Ben is a horse master, and loves horses, so when the Moss' take the young boy in, they decide to give him work at the neighbors house driving cows (on a horse, of course). After that a series of events happens, and Ben finds out his beloved father is dead. Miss Celia, a neighbor, feels sorry and comforts him, and finally offers to let Ben stay with her and her fourteen-year-old brother, Thornton who is called Thorny. After that many adventures and summer-happenings go on in Celia's house. Sancho gets lost, Ben is accused of stealing, Miss Celia even gets hurt and Ben takes a wild ride on his horse, and… The rest you'll know from reading the book.
Summary by Wikipedia, revised by Stav Nisser.