A typo in equation (6.24).

The closed loop transfer function $G(s)$ (which means $H_{yr}(s)$) should be $\frac{{\omega }_0^2}{s^2+2\zeta {\omega }_0+{\omega }_0^2}$. The last term of the denominator is ${\omega }_0^2$ rather than ${\omega }_0$.

--Shuo