Tree House 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 Alden used to live alone in a boxcar. Now they have a home with their grandfather and are helping their neighbors build a tree house.

From the new tree house, the Aldens can keep an eye on the house next door. That house has a very old and special secret...and the Boxcar Children are determined to find out what it is!