Everyone in Room 3B is excited about their assignment to study the moon. So when Harry finds an ad for a used telescope, the class decides to have a bake sale to raise the money to buy it. Once they get the telescope they plan to have a moonwatch and invite all the parents--but no one knows that Harry has a surprise plan of his OWN for the big night.

The kids in Room 3B are holding a Sharing and Caring Tag Sale to earn money for charity. Song Lee and Ida are selling friendship bracelets, Doug is selling books, and Sid is selling delicious, fresh-baked chocolate chip cookies. But something else is on Harry's mind: his grandmother is having money trouble, and Harry wants to help. So he comes up with a plan to raise money for her. The problem is that Harry doesn't realize his plan involves stealing from the class's charity collection! How can Doug stop his friend from becoming a horrible thief? Could the answer come from one of Sid's delicious cookies?

For use in schools and libraries only. Inspired by his third-grade class's study of the moon, Harry announces that he is going there.

Another fun mystery starring Horrible Harry, and this time he's calling the shots! Harry has always loved solving mysteries, and now his friends in Room 3B have learned to love it, too. So Harry challenges them to figure out what he has hidden in his giant, old suitcase. Led by a very determined Mary, the kids take turns solving clues and piecing the mystery together. Their hunt leads them from the classroom to the library, and even to Harry's grandfather's nursing home. Will they be able to solve Harry's mystery? Or will things go horribly wrong for the novice detectives?

Doug discovers that though being Harry's best friend in Miss Mackle's second grade class isn't always easy, as Harry likes to do horrible things, it is often a lot of fun.

Harry leads a mission to place cobwebs all over school, shows the class how to make green slime, and stages a "deadly skit" that has everyone on the edge of their seats.

The kids in room 3B have found out their teacher is engaged! But who’s the lucky groom? Harry goes into detective mode to find out, and Doug is willing to help—until he sees Harry stealing a piece of evidence! Doug can’t believe what Harry has done, but Harry thinks his risky move is worth it. After all, he cracks the case and tells everyone who Miss Mackle is going to marry . . . but does Harry have his facts straight? Or has he made a horrible mistake?