From electricity to fiber optics.

Petzold's parting thoughts in Code are well worth noting here:

While much of this book has focused on using electricity to send signals and information through a wire, a more efficient medium is light transmitted through optical fiber —thin tubes made of glass or polymer that guide the light around corners. Light passing through such optical fibers can achieve data transmission rates in the gigahertz region —some billion of its per second.

So it seems that photons, not electrons, will be responsible for delivering much of the information of the future into our homes and offices; they'll be like faster dots and dashes of Morse code and those careful pulses of blinking light we once used to communicate late-night wisdom to our best friend across the way.

(Petzold: p. 382)

Yet, in spite of all these technological developments, the core of his book remains relevant. He tells us about the main concepts in electrical engineering in order to introduce us the key concepts of computer science without which we would miss the foundations of this particular discipline of knowledge. However, the concepts we learn along the way (rudimentary concepts of information, binary and hexadecimal languages, circuits, addressing memory, etc.) are still as valuable today as they were 20 years ago.

This is an excellent book. It's highly recommended to anyone who cares to know about how things work in the world of computing.