The Tree House Mystery, by Carol Beach York (epub/Kindle/pdf)
When Roger first looked up at the tree house, he thought he saw something moving at the window. No one should be there -- the tree house belonged to the Mayfairs. It was in the deep woods that surrounded their new house in the country. Still, strange things happened each time Roger and his sister, Anabelle, tried to explore the tree house. Once Roger heard mysterious noises. Another time they found odd-shaped footprints. What was the secret the tree house was hiding?