Terrific introductory book about computers. Petzold assumes the reader knows nothing and explains everything in a clear simple way from the very bottom level of functional abstraction up, in a building logical progression. Learn how bar codes work, how a lightbulb (or other output) can "answer" which switches are closed in a circuit, what "and" and "or" gates are and how to use these to build more complicated gates, and how to build a binary adding machine.