An American fairy tale, on purpose
L. Frank Baum said plainly what he was doing. In his introduction he wrote that the old European fairy tales were full of "heartaches and nightmares" — the cut-off toes, the ovens, the punishments — and that he wanted to write a modern one for American children that kept the wonder and left the horrors out.
You can feel him doing it. Nobody in Oz is punished for being foolish. The Wizard turns out to be a fraud and is treated kindly anyway. The book is uncommonly good-tempered for its period, and that is a deliberate act of authorship rather than an accident.
The shoes were silver
This is the detail worth knowing, because it tells you how much the 1939 film rearranged.
Dorothy's shoes in the book are silver. The ruby slippers were invented for the film, reportedly because red looked better in Technicolor. Silver shoes on a yellow road, in a book written when America was arguing furiously about silver money, is a detail people have been reading meaning into for a century.
The film changed more than footwear. In the book Oz is a real place that Dorothy really goes to; the film makes it a dream she wakes from, which is almost the opposite of what Baum wrote. The Witch is not green. There are no songs.
None of that makes the film worse — it is a wonderful film. It just is not the book, and if you are reading the story to a child it is worth knowing which one you are holding.
What the story is actually about
Three travelers walk a very long way to ask a wizard for a brain, a heart and some courage.
Along the road the Scarecrow solves every problem the group meets. The Tin Woodman weeps over a beetle he stepped on. The Lion walks straight at every frightening thing they encounter while being frightened the entire time.
Then they arrive, and the Wizard is a small man from Omaha with no magic at all, and he gives them bran, a silk heart and a green drink — and it works, because the only thing any of them was ever missing was somebody telling them what they had already been doing for weeks.
It is a kinder idea than "believe in yourself," and a more useful one. You are often already the thing you are trying to become. Someone usually has to point it out.