newtonian mechanics - Work-Energy theorem and System

In the book Classical Mechanics by David Morin,while discussing Work Energy theorem he gives an example of importance of choosing System. As he considers lifting the book upward then by WE theorem:

$W_{external}$= ∆K+ ∆U +∆Internal Energy

Now,if we consider lifting the book and choosing only book as System,we apply WE to get:

$W_{person}+W_{earth}=0+0+0$

Now,I understood that change in K.E and Internal energy of System is 0 but why change in potential energy is 0 if we keep book as system.

Answer

You sure this is the exact statement. Because $\Delta U = -W_{Earth}$ is the definition of gravitational potential energy. You should never include both of them in the same expression.

Either this or the fact that since you are only considering the book as your system the potential energy of the system(book only) does not get changed at all. What is getting changed is the potential of the system containing earth and book.