The story of a young man who finds out, after many adventures as a boy, that he’s a girl and accepts that fact and goes on to rule Oz as it’s beloved queen. The book is noteworthy for that alone. Otherwise, it’s a fun little book. No real notable characters that I really love. Jack the Pumpkinhead and the Woglebug are the only lasting additions and neither one of them is particularly memorable to me.
I do think this is notable for the way Baum changes his world around to suite the book he's writing. For example, her Momby is a decent witch but nothing special. In later books she becomes one of the wicked witches who ruled Oz after their king died. Also interesting, the way the Wizard becomes a villain here- spiriting away poor baby Ozma to Momby so that he could rule in peace and in later books it's simply that Momby stole the princess and the Wizard had nothing to do with it. It could be that Baum was playing with unreliable narrators and rumors but I'm inclined to think he wasn't really paying a lot of attention to continuity.