The Secret of the Strawbridge Place by Helen Pierce Jacob, 1976.
This story takes place in Ashtabula, Ohio during the Great Depression. Kate is frightened of the hobos who pass through town looking for work, but at the beginning of summer, her brother Josh dares her to come with him to spy on the hobo camp. The two of them witness a fight between three hobos, and in their haste to get away, Kate falls and breaks her arm. At first, she is sure that her summer is ruined, but when she considers the place where she fell, she realizes that she has stumbled on an important clue to a secret surrounding the old house where they live.
Locals say that during the Civil War, the Strawbridge family, who lived in the house before Kate’s family, were part of the underground railroad, hiding runaway slaves. However, no one has ever been able to find the place where the slaves were hidden. When Kate fell, she discovered the opening to a cave near the river that she never knew was there before.
Oscar, a boy visiting his grandfather nearby, becomes Kate’s friend. Since he was also injured in one of Josh’s escapades (having broken his leg when the kids were fooling around in the haymow), she invites him to join her in the search for the secret. They form a partnership called Cripples Incorporated and have fun inventing codewords and writing secret messages about what they’ve discovered. Pursuing the secret comes with some risks, and before Kate can discover the whole truth about Strawbridge Place, she has a serious brush with danger.
It’s an interesting mystery that invites readers to try to figure out the clues along with Kate and Oscar as they ponder the sampler with the strange motto left behind by the Strawbridge twins. Oscar also introduces Kate to Sherlock Holmes stories, one of which provides her with the inspiration to solve the mystery. Kate also develops better feelings for the hobos, who, like the runaway slaves, turn out to be mostly ordinary people just looking for a better life.