A little girl remembers the donkey her father bought for her and her sisters. The donkey would only go so far, and would then lie down in his harness. The girls had to wait until he was ready to go on again. It was such a long wait they would take books along with them on journeys to guard against the boredom of waiting.
Mary Ellen Chase (1887-1983) was educated at the University of Maine. After gaining her PhD, she became an Assistant Professor at the University of Minnesota. She wrote literary criticism and bible studies as well as fiction. Much of her fiction was set in Maine. She wrote, as far as I am aware, one equine-related book, the story of a very awkward donkey.
Finding the book: the hardback version is rather more expensive than the paperback, but not impossibly scarce.