As a hobbyist programmer, I had wondered for years how a computer worked at its most internal level. I had even had a couple of unsuccessful encounters with other "How Computers Work"-type books that left me mainly with the dissatisfied impression that the machine was an impenetrable black box.
Petzold's book was not like this. While other books started with flashy graphics of internal designs all flowcharts with little in the way of explanation, Code starts off simply, with two flashlights and the goal of communicating at night. This problem, of course, would be easy solved by anyone who knows of Morse code. Yet, from Petzold's pen it becomes an illuminating and amusing journey with attempts to deal with similar scenarios of every increasing complexity until I realized two-thirds of the way through that given a sufficient time and space, I could, at least in theory, build a computer. This is the greatest success of Code, in my opinion. Rather than attempting to peel away the mysteries of the system a layer at a time like an onion, it delves directly to the core and builds upon its ideas in a fashion like that of the original computer designers, until everything forms into a cohesive whole. Petzold does an excellent job of capturing his excitement for the material, making the progressive developments a joy to read about.
With my uneven background knowledge, there were a few sections that I felt that I could skim through, but also many a passage that I had to carefully scrutinize. Overall, I'd recommend this book for anyone interested in computers, from beginner to expert: Petzold provides enough explanation for a determined newcomer to understand all of the concepts, but enough breadth to still entertain and educate those with more experience in the area.