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?