This is an outstanding primer on the world of graphic design. It's written for ages ~12 and up, but adults can benefit from the content as well (particularly font junkies).
It's split into the sections of Form, Typography, Content, and Concept, with 10 self-directed projects to do at the end. Kids are encouraged to post their finished work on GoTheBook.com
, which caps the interactive tone of the book very well.
Highly recommended for teachers, students, and anyone interested in why things look the way they do.