The "Impedance" Equation
The book is downloadable from here: Right First Time and the relevant section of the book is pages 119 to 122.

In this book the author is specifically talking about PCB design and is making the reasonable assumption that \$ R_0 \$ and \$ G_0 \$ and negligible in comparison with the inductive and capacitive effects for frequencies of a few 100kHz and above.

This fits with my personal experience of PCB design and several authors have made the same assumption.

The statement \$ Z = \sqrt{\dfrac{R_0 + \text{j} \omega L_0}{G_0 + \text{j} \omega C_0}}\$ is false but at these frequencies the impedance varies very little with frequency and \$ Z = \sqrt{\dfrac{L_0}{C_0}}\$ is a very close approximation.