The Haunted House by Peggy Parish, 1971.
The Roberts family is moving to a different house across town because they want a bigger place to live. Liza, Bill, and Jed are not very happy about moving, at least at first, because they will be farther away from their friends, especially Jimmy and Mary, who have been their best friends and next door neighbors. Liza is also worried because the house they will be living in is the old Blake house, which is rumored to be haunted. A couple of past caretakers have left because they claimed to hear ghostly noises. The boys tease her about it, and their father insists that there is no such thing as ghosts.
The house is definitely much bigger than their old one, and there is a barn, a garden, and a large wooded area on the property. When they actually move in, the kids become fascinated with the old things in the attic, which the children are allowed to go through. The kids also plan to start building a tree house. The boys become excited about the house, but Liza is still nervous about the ghost stories.
Then, strange notes and coded messages appear for the children, starting with a note taped to Liza’s window in the middle of the night. Little presents are left for them in a hollow tree with promises of bigger surprises later. Who is sending these messages? Could it be the ghost?
Like other books in this series, there are coded messages and word games that the readers can solve alongside the children. Some of them are easily recognizable, like word scrambles and Pig Latin, and others are a little harder. The kids walk readers through their solutions, but readers can attempt the puzzles on their own first.