Hello!
If the book is now on the floor (below the shelf), its potential energy is 0 J.
Potential Energy is defined as the energy that is capable of generating work as a consequence of the location of the object.
When the book is on a shelf, it is capable of generating work by falling from the shelf onto the floor by the force of gravity. This is called gravitational potential energy.
When the book is on the floor, it is no longer capable of generating work by falling, and its potential energy is 0 as it can't fall anymore and generate work.
Have a nice day!
