Most people probably know about Mary Poppins from the 1964 Disney film starring Julie Andrews and Dick van Dyke. A more recent film, Saving Mr Banks starring Tom Hanks and Emma Thompson highlighted the gulf between the "Disney version" and author PL Travers' vision of the story.
Members of the book group all noted the difference between the saccharin Mary Poppins of the Disney movie and the sterner, much vainer Mary Poppins in Travers' books (Mary is for ever admiring her reflection in shop windows), where the children are often reprimanded for mentioning their nanny's magical abilities, and are constantly found wanting in their appearance and behaviour.
The episodic nature of the book also appealed to members of the group. Each chapter is almost a self-contained short story ( a structure which influenced Neil Gaiman in writing the Graveyard Book).
If you've only ever seen the film and felt it could do without that extra "spoonful of sugar", it's time to give the original stories a try.