[Maximusace9]

You can also see that each slot determines part of the rider based on how much is displayed
So, with the Brave Dragon WonderRide Book in the rightmost slot, and nothing else, the main body of Saber and the right arm are set.
With Peter Fantasista or Jack and the Beanstalk open in the leftmost slot, they show the arm, covering the middle slot.
So, with the Hidden Story of Hiden on the Left, A.I.M.S. Animal File in the Middle, and Metsuboujinrai Monsters on the Right, the full set of books, when opened will display, from Left to Right, Zero One's left arm, Vulcan's Body, and Horobi's right arm.