We used to have a guy at work who, whenever such things came up, would declare that the 1939 movie was the best adaptation of book-to-film ever. I'd say "Interesting. I've never read it - how were the musical numbers described in the book?"
I know, right? The 1939 movie, while enjoyable in it's way, is probably one of the worst book to movie adaptations. It'd be like going to see the first Harry Potter movie but Harry is 25 and it's a musical. There's just so much that's different, but I'm not really sure how else they could've streamlined it for the screen.
"Baum screwed around with Baum" from one book to the next.
He had a right to do so: it was his own stuff.
And his stuff is now in the public domain for anyone to do what they please with as is their