The Mystery of the Secret Message — Gertrude Chandler Warner

One warm night four children stood in front of a bakery. No one knew them. No one knew where they had come from.

Henry, Jessie, Violet, and Benny used to live alone in a boxcar. flow they have a home with their grandfather, and they're helping him organize Greenfield's Winter Festival.

When the Boxcar Children get a roll of photos developed, they find a secret message hidden in one of them. The Aldens are baffled, and things become even more mysterious when they realize that someone is trying to ruin the Winter Festival by tearing down the decorations and stealing a local statue! There must be a connection between the message and everything that is going wrong. Can the Boxcar Children figure out what it is in time to save the Festival?