Horrible Harry and the Dead Letters

The kids in Room 3B can't believe their luck! The post office has donated a real mailbox to their classroom, and Harry and his friends are excited to learn how mail is delivered. Miss Mackle assigns everyone a job - Mary is in charge of the mailbox key, while everyone else pitches in as sorters, stackers, and carriers. ZuZu handles the misaddressed - or dead - letters. But when several kids mysteriously have their bookmarks disappear, Harry discovers that a thief is using the mailbox for something other than letters! Will he sort out what's going on? Or will Harry deliver some horribly wrong conclusions?