My kids (ds11 and dd 9) love Larry Gonick's cartoon guides (Physics, Genetics, world history, American history etc.); there's one on computers, too -

http://www.amazon.com/Cartoon-Guide...mp;ie=UTF8&qid=1302786597&sr=1-1

It describes and illustrates the basic principles of processing units, binary code, elementary logic, etc. and summarizes the history of computer development (Babbage, Lovelace, Hollerith, Turing).