the boxcar children: the mystery bookstore

After their Grandfather has bought a bookstore and rented it to his friend, Henry, Jessie, Violet and Benny are helping out. As a reward, they each get to pick a book to keep. But the Boxcar Children find mysterious clues of a person entering the store and even their rooms at night, unseen. Then Violet's books disappear. Can they get to the bottom of this?