The 21 Balloons by William Pene du Bois

HC, Tenth Printing 1967

Professor William Waterman Sherman intends to fly across the Pacific Ocean. But through a twist of fate, he lands on the secret island of Krakatoa where he discovers a world of unimaginable wealth, eccentric inhabitants, and incredible balloon inventions. Winner of the 1948 Newbery Medal, this classic fantasy adventure is a joy for all ages.