Buildings have walls and halls.
People travel in the halls not the walls.
Circuits have traces and spaces.
Energy travels in the spaces not the traces.
A word about the book title
Mach 1 was a barrier in flight for a long time. Aircraft that can go faster than the speed of sound are more expensive and more difficult to design. There is a barrier in digital design that occurs at clock rates around 1 GHz. One clock period is one nanosecond, and in this time an electromagnetic wave can travel about 15 cm in epoxy. This is the dimension of a typical circuit board. Circuits that can perform near or above 1 GHz present a new set of design challenges. In that sense, there is a barrier to cross. This book discusses the challenges of designing these faster and faster circuits. Many old ideas must be discarded and new ones accepted. There are no sonic booms that I know of. I hope the ride through Mach 1 is a smooth one.