Mandie and the Secret Tunnel by Lois Gladys Leppard, 1983.
Young Amanda Shaw is almost twelve when her father dies, leaving her with a bully of a sister and a mother who doesn’t really love her. Her only friends are Joe, who is the son of the local doctor, and Uncle Ned, a Cherokee Indian. Amanda and her father are part Cherokee, and Uncle Ned helps to look after her now that her father is gone. Then, Amanda’s mother remarries and sends her to live with another family who want someone to help them look after their young son. However, the family is rather mean to Mandie, exploiting her for cheap labor, and Uncle Ned offers to take Mandie to live with her uncle in the town of Franklin.
Mandie never knew that her father had a brother (Uncle Ned is Mandie’s “uncle” by affection, not blood), but she readily agrees to go to him. Uncle Ned and some other Cherokees help Mandie to sneak away in the middle of the night and make the journey to Franklin. When she arrives at her uncle’s house, she is taken in by the servants, but she is told that her uncle is away in Europe. Mandie’s uncle turns out to be rich, and Mandie is very happy in his house. She gets new dresses for the first time in her life and makes friends with Polly, the girl next door.
Then, word comes that her uncle has died in Europe. Her uncle’s will is missing, and no one knows exactly what will happen to her uncle’s property or to Mandie until it is found. Strangers come to the house, saying that they are also relatives of Mandie’s uncle, but Mandie wonders if they really are who they claim to be. With the help of Polly and Joe, who comes to visit her, Mandie searches for her uncle’s missing will and accidentally finds a secret tunnel leading out of her uncle’s house.
As Mandie and her friends investigate this mysterious situation, where things and people aren’t what they claim to be, Mandie learns that her own past isn’t what she believed it was. Her family has kept many secrets from her, and learning these secrets leads her to a better future than she’d ever imagined.