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