The Invisible Intruder is the 46th volume in the Nancy Drew Mystery Stories series. It was first published in 1969 under Carolyn Keene. The actual author was ghostwriter Harriet Stratemeyer Adams.
"Nancy Drew, forget the ghost hunt!" a male voice rasps on the telephone. Despite the mysterious warning, the teenage detective and a group of friends start out on a ghost-hunting expedition, investigating five places reputed to be haunted. Danger strikes at once when Nancy tries to overtake her canoe that paddles itself on Lake Sevanee. The ghost hunters then pursue a phantom horse and its ghost rider that runs across the field surrounding Red Barn Guesthouse. During these events, Nancy finds herself in a dangerous adversary, clever enough to operate invisibly. In a dramatic climax, Nancy outwits her enemy in an eerie mansion and traps him/her in the fantastic Room of Skulls.