An object kept at a height has a potential energy due to the gravitational force of attraction of the Earth. This potential energy is acquired as work has to be done to raise the object.
The gravitational potential energy is equal to m*g*h, where m is the mass of the object, g is the acceleration due to gravity and h is the height.
The book weighs 12.0 N. This gives m*g = 12 N.
When the book is kept on the table 75 cm above the floor it has a potential energy of 12*0.75 = 9 J.
On the shelf which is 2.15 m above the floor the potential energy is 12*2.15 = 25.8.
This gives the change in potential energy when the book is raised from the table and placed on the shelf as 25.8 - 9 = 16.8 J.