The Mystery Horse — Gertrude Chandler Warner

One warm night four children stood in front of a bakery. No one knew them. No one knew where they had come from.

Henry, Jessie, Violet, and Benny used to live alone in a boxcar. Now they have a home with their grandfather, and they're spending their vacation on a farm.

The children learn to do everything from milking cows and making homemade jam to raising a barn. Then one day they spot a beautiful horse locked up in the stable. Who is this mysterious horse? And why is it hidden?