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?