What is the point of view in the book The Twenty-One Balloons?
The point-of-view of this story actually shifts several times. If you include the Introduction as part of the story, the point-of-view is a mixture of 3rd person and 2nd person, as the speaker identifies the audience directly (by using "you") several times.
Chapter one then opens using a 3rd person, but seemingly limited point of view. It is limited, because the details of Sherman's adventure are not included, and in fact, presented as a mystery. This chapter prepares the reader to hear the story of Sherman and his balloon, from Sherman himself.
In chapter three, the point of view shifts from 3rd person to 1st person, as Sherman becomes the narrator of his own story. The reader of the book joins the audience of townspeople, the mayor, and of course the members of the Explorers' Club, in listening to Sherman (speaking from a bed) tell his tale. The rest of the story continues in this voice.
Then, in the final chapter of the book (and in fact, in the final five paragraphs), Sherman finishes his story with "If you have any questions I'll gladly try to answer them." Here, the point-of-view shifts back into third person and the perspective pans out over the audience where someone asks Sherman how he could tell such a story in his condition. He jumps from the bed, announces that his health was restored on the long train ride from New York to San Francisco, and that he could have told the story standing up. When he saw the bed, he thought he'd be a fool to pass it up. The story ends with Sherman announcing his plan to rebuild his balloon and spend a year traveling inside.