The Deserted Library 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. Now they have a home with their grandfather and are helping to save an old library that's about to be torn down.

The children find several mysteries at the library. Someone is breaking in and stealing their food. Someone, long ago, hid pieces of an old Civil War sword. And after all the children's hard work, someone is trying to destroy the library. Who?