I have a BS in Computer Science. I have had 12-20 undergraduate hours containing at least some of the material in the book. It starts off on really simple stuff, like understanding the basics of electricity, then it moves on into using switches as gates, and eventually into how relays work. From there, you build (well, conceptually at least) gates, then your first flip-flops, and then you rapidly move into turning all of these ideas into a concept of a real and working computer. I thought the last couple chapters (on floating point and GUIs) were incredibly boring compared to the rest of the book. They are admittedly dry topics in my estimation.
Petzold's bottom-up explanation of the basics of computer architecture is amazingly good. I learned some things and a lot of what I thought I knew was solidified.