There is more than one theme present in this book. The majority of the book is about the Professor's time on the island of Krakatoa, its incredible population of people, and the society that they have built. They have more or less built a utopia on a volcanic island, and they have managed to keep it a secret. The people on the island are all incredibly intelligent and hardworking people, and this shows off a theme that emphasizes the importance of education. Additionally, the people are not selfishly using their ingenuity and hard work. The community is exceptional for their altruistic behavior. This book shows young readers a wonderful theme about community and the importance of helping out your neighbors. Creativity and individualism are also emphasized in this book. The island society strongly supports these kinds of attitudes and behavior. A bit of evidence that supports this is the balloon merry-go-round. The entire thing was conceived by the children, and the adults were very helpful in allowing the kids to pursue this particularly creative piece of entertainment.

The Twenty-One Balloons stresses the themes of creativity, individuality, eccentricity, and cooperation. The fantasy of The Twenty-One Balloons is built around an actual historic event—the massive volcanic eruption that destroyed the Pacific island of Krakatoa in 1883. But there the connection with history ends. The Professor discovers that the inhabitants of the island have established a unique, Utopian society, which he seeks to understand. The story begins when Professor Sherman, who teaches high-school, becomes bored with his life and sets off on a journey in a hot air balloon called The Globe.

He hopes the wind will blow him and his balloon all around the world. To his surprise, he instead has a crash landing on the mysterious island of Krakatoa, an island full of diamond mines and enormous wealth.

The secret society of Krakatoa is based on values of greediness for wealth and inactivity. They believe their lives are perfect because they never have to worry about money. They live empty and unfulfilling lives and must learn the value of relationships, education and their own lives. They must learn how having extreme excess of money, or anything, is worthless.

Threatened with destruction, the Professor and the inhabitants must cooperate and discover a way to escape the island before the final explosion.

What is the author's message in The Twenty-One Balloons?

I think this question is asking about something other than possible themes. It is possible that the author had several messages in mind. One possible message is that the author was trying to show people what a society could be like if everybody worked together and behaved much more altruistically than currently done. It's also possible that the author was trying to send a message about the value of art and science working together, suggesting that they should do so more frequently.

Personally, I feel that the message is squarely aimed at taking mankind's power down a peg or two. The people on the island are amazing. They are friendly and intelligent, and they invent all kinds of amazing technology; however, none of that could prevent the destruction of everything that they built. Escape was possible, but their homes and lives on the island have been destroyed. The message could be something along the lines of "nature always wins," and it shows readers that nature will always be more powerful than humanity.

What is the author's message in The Twenty-One Balloons?

Pene du Bois published this book in 1947, so long ago that we might have a difficult time understanding the context in which he wrote. One idea that comes to mind is that he was poking fun at Utopian societies in the book, and perhaps making the point that no matter how perfectly we set up societies, nature has a way of making things imperfect. You might also notice that the book was published almost immediately after World War II, so it is equally possible that he longed for a peaceful island on which people could create an ideal and happy society. In either case, a similar eruption really did happen on the island of Krakatoa, and whether the volcano stands as a symbol of the damage to civilization in World War II, or a symbol of nature's way of gumming up the plans of people, is something we may never know.  It is equally possible, given that the author wrote many children's books, that he simply wanted to write a great adventure story, which he most certainly did!