The Boxcar children: Surprise Island by Gertrude Chandler Warner, Scholastic Books 1989
Back cover shows shelf wear, Inside clean and tight. Boxcar children series Book 2.
Henry, Jessie, Violet and Benny used to live alone in a boxcar. Now they have a home with their grandfather and are spending the summer on their own private island. There is a mysterious stranger on the island with a secret--but that's not the only surprise in store for the Boxcar Children