The Disappearing Friend Mystery — 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, but now they live with their grandfather. And they've just made a new friend named Beth.

The Boxcar Children decide to raise money to help the Greenfield hospital. At Beth's suggestion, they form a service called The Boxcar Helpers -- to baby-sit, wash cars, mow lawns, and care for pets. But Beth keeps disappearing. And then other things start to go wrong. Is someone trying to put them out of business?