If you would like to start reading the Wizard of Oz series but not sure where to start, then list below might help. In April 2018 informal poll of the Oz books written by the original Famous Forty authors by International Wizard of Oz fans has concluded with the results shown below based on popularity of the titles.
Ozma of Oz 56.21
The Patchwork Girl of Oz 44.80
The Wonderful Wizard of Oz 40.92
Glinda of Oz 37.17
The Lost Princess of Oz 37.00
The Tin Woodman of Oz 37.00
The Runaway in Oz 35.91
The Wonder City of Oz 34.98
The Road to Oz 32.00
The Emerald City of Oz 32.00
The Magical Mimics in Oz 31.8
The Scarecrow of Oz 28.08
Tik-Tok of Oz 25.20
The Lost King of Oz 22.94
The Forbidden Fountain of Oz 22.88
The Cowardly Lion of Oz 22.12
Jack Pumpkinhead of Oz 22.12
The Magic of Oz 21.83
The Shaggy Man of Oz 21.2
Merry Go Round in Oz 21.12
Lucky Bucky in Oz 21.09
The Marvelous Land of Oz 21.08
Kabumpo in Oz 21
The Wicked Witch of Oz 20.1
The Scalawagons of Oz 18.02
Ozoplaning with the Wizard of Oz 18
The Enchanted Island of Oz 17
Dorothy and Wizard in Oz 16.79
The Wishing Horse of Oz 16
Ojo in Oz 14.96
Handy Mandy in Oz 12
The Purple Prince of Oz 11.97
The Gnome King of Oz 11
The Hidden Valley of Oz 9.9
Pirates in Oz 9.03
The Royal Book of Oz 9
The Giant Horse of Oz 9
Captain Salt in Oz 9
The Hungry Tiger of Oz 8.06
The Silver Princess in Oz 8
Rinkitink in Oz 7.92
Speedy in Oz 7.04
Yankee in Oz 7
Grampa in Oz 5.88
The Yellow Knight of Oz 5.88
The Cheerful Citizens of Oz 3