I have no idea what the title is but the book would appeal to a 8-10 year old girl. It was in my grade school library. I remember that the girl was orphaned or sent to live with a relative, possibly a grandmother or aunt. I’m leaning toward aunt. The place is mysterious and there is a mystery of sorts. The key part I remember is the young girl protagonist finds a secret note or message or clue on paper under a loose brick, I believe around the pool or on the patio, but I’m 99% sure it was around the pool. This would have been around 1978-80, although the book may have been slightly older.
Stump the Bookseller
New format, same book stumper solving.
Possibly Phillis Whitney’s Mystery of the Haunted Pool
http://www.phyllisawhitney.com/Mystery%20of%20the%20Haunted%20Pool.htm
The Secret Garden? maybe.. though it’s not a note but a hidden key..
Possibly The Ghost Next Door by Wylly Folk St. John? Here’s a link to the Goodreads page on the book:
http://www.goodreads.com/book/show/464102.The_Ghost_Next_Door
One of the things that the girl discovers is a note behind a rock bordering the pond behind her grandmother’s house.
The cover art on the Goodreads page is the second cover art, which looks oddly like a horror novel….very misleading. The original was by Carnegie Award winning Trina Schardt Hyman and the style was completely different
This is a long shot, and the details don’t quite fit but any chance this is “A Spiderweb for Two” by Elizabeth Enright? I seem to remember one scene where the clue for the scavenger hunt was taped to a brick wall with ivy over it.