Why is the first ionization enthalpy of zinc higher than expected? The following sentence is stated in my current chemistry book:


  The first ionization enthalpy of zinc is higher because it represents an ionization from the $\rm 4s$ level. 


Now zinc does have an ionization enthalpy of $\pu{906 kJ/mol}$, which is higher than copper's $\pu{745 kJ/mol}$ or nickel's $\pu{736 kJ/mol}$ in the same series. But I am not sure that the reason for this increase stated in the book is correct. For example, even copper has an ionization from the $\rm 4s$ level, but it does not have anywhere near as high ionization enthalpy as zinc. Is the book correct, or is there some other reason for this?
It is because the shell is complete.

Zinc's electronic configuration is $\rm [Ar] 3d^{10} 4s^2$, which is a complete configuration for a transition metal. The next element in the same period is $\ce{Ga}$, which starts filling $\rm 4p$ orbitals ($\rm [Ar] 3d^{10} 4s^2 4p^1$).

The zinc's electronic configuration causes a sort of special stability because of the filled electronic states. Also, the effective nuclear charge is greater than $\ce{Cu}$'s, so it's more difficult to ionize $\ce{Zn}$.