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.