The Twenty-One Balloons

I've been looking for this book for decades! I remember reading it in somewhere around high school and it's stuck in my mind to reread it as it had some really unusual inventions in it. Now that I've found and reread it, it's still pretty amazing.

It's written for grade schoolers but references to Krakatoa (which I didn't know about then but do now), predictions on the new technologies (copyright 1940s), and the main character's desire to lead a simpler life (which I now want as well) really held my interest.