The Message in the Hollow Oak (Nancy Drew, Book 12)

A group of professional detectives challenge Nancy to tackle a mystery that they have failed to solve: find an invaluable message hidden by a missionary centuries ago in a hollow oak tree in Illinois. While searching the woods for the ancient tree, Nancy and her friends live with a group of young archaeologists who are excavating prehistoric Indian burial mounds on a nearby farm. A shadowy enemy stalks Nancy and harasses everyone at the dig. The young investigator pursues her dangerous adversary to an outlaws’ cave, and is threatened when she discovers an unusual treasure. How Nancy, with few clues to go on, solves this complex mystery will thrill all readers.

• The Clue of the Broken Locket (Nancy Drew, Book 11)
• The Mystery of the Ivory Charm (Nancy Drew, Book 13)
• The Password to Larkspur Lane (Nancy Drew, Book 10)
• The Whispering Statue (Nancy Drew #14)
• The Haunted Bridge (Nancy Drew, Book 15)
• The Sign of the Twisted Candles (Nancy Drew, Book 9)
• The Clue of the Tapping Heels (Nancy Drew, Book 16)
• The Mystery of the Brass-Bound Trunk (Nancy Drew, Book 17)
• Nancy's Mysterious Letter (Nancy Drew Mystery Stories, Book 8)
• The Clue in the Diary