This book is literally one big non-sense birthday party. It's story line is a collection if incredible fun events.
This is probably the 'silliest' book on this list, however, it is so artfully written that the ridiculous takes on artistic significance and becomes worthwhile.
The best thing about the book is Mary Poppin's character. She is the most wonderful nanny the children could have wished for, taking them on remarkable adventures. Yet she is comically strict and pompous. The paradox of being strict and uptight with being amazingly fun is what characterises both the storyline and the book's main character.
You could say that the plot of this book is to ridicule etiquette whilst at the same time upholding it.
This is not a very deep book, I would call it silly, but it is very well written and so its silliness becomes 'fun'.