The Wonderful Adventures of Nils
I have enjoyed Lagerlöf's books of legends, and for a long time I've meant to read this story. (After all, my oldest got it from the library and read it at age 5, so surely I can do it...) She wrote it as a geography textbook for Swedish schools, published in 1906–07, and in the English edition, some of the longer geography passages are edited out. This turns it into a long adventure story, still featuring a whole lot of Swedish geography.
Nils Holgersson, age about 14,* is kind of a stinker of a boy. He's lazy, rude, and mean to animals -- a real worry to his parents. And one day, he spots the house elf, catches him....and the elf gets his revenge by turning Nils into an elf too! Nils can now understand and speak with animals, but they don't think much of him. What to do?
He winds up taking a trip with the wild geese, who are migrating north to Lappland for the summer. They call him Thumbietot, since he's so small; just the right size to ride on a goose. On the way, he has many wild adventures, and soon learns to be a helpful and kind, especially to animals. He earns himself a good reputation, and travels all over Sweden, seeing many wonders and learning the importance of leaving space for wild animals. And although it takes him some time, he eventually realizes that his parents probably miss him.
Nils' adventures take up two volumes, and before reaching home, he has to solve a terrible moral dilemma. In the end, old Akka, the wise leader of the geese, says to him:
"If you have learned anything at all from us, Thumbietot, you no longer think that the humans should have the whole earth to themselves...Remember you have a large country and you can easily afford to leave a few bare rocks, a few shallow lakes and swamps, a few desolate cliffs and remote forests to us poor, dumb creatures, where we can be allowed to live in peace."
*In Scandinavia, you get confirmed at 14, and in the old days, this marked your passage to adulthood. So -- Nils is definitely old enough to know better.