The Clue in the Cobweb — Carolyn Keene

Louise and Jean help Captain Dana find Katherine Blore, a woman who disappeared mysteriously from his ship before it docked. The Danas trace Miss Blore to her current residence, but she refuses to see them. The Danas try once again to see Miss Blore but find her home empty except for the landlord, who is locked in a closet.

The landlord, Mr. Wharton, has suffered a heart attack, so Louise and Jean help him get the house rented by showing it to prospective tenants. While in the house, Louise falls strangely unconscious and remains asleep for 12 hours. Later, the same fate befalls Mr. Wharton and one of the new tenants.

Louise begins researching herbs, thinking that a poisonous herb may be the culprit for the sleeping illness. The girls divide their time between searching for the mysterious herb and searching for Miss Blore, who has left the state. The girls must travel to a distant ranch and back home again before they can solve the mystery.