The Old Motel 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 they've gone on a vacation with their Aunt Jane.

Aunt Jane takes the children to stay with her friend, Kay, who owns an old, shabby motel in Florida. Kay is ready to sell the motel, when the children offer to help fix it up and put it back in business. But they soon learn that someone is trying to make sure it stays out of business...forever.