Nancy Drew: Password to Larkspur Lane by Carolyn Keene

Book number 10 of the Nancy Drew Mystery Stories series

Blue bells will be singing horses! This strange message, attached to the leg of a wounded homing pigeon, involves Nancy Drew in a dangerous mission. Somewhere an elderly woman is being held prisoner in a mansion, and  Nancy is determined to find and free her. Meanwhile, the young detective’s close friend, Helen, begs her to solve a second mystery. Helen’s grandparents, the Cornings, are frightened by a sinister wheel of blue fire that appears after dark in the woods outside their home at lonely Sylvan Lake. When Nancy discovers the significance of the eerie signal, she also learns that her two mysteries are connected.