I think for anybody that has a passing interest in comics, the creative process, or even art in general, Scott McCloud's Understanding Comics is essential reading. McCloud made the ingenious decision to explore the theory and workings of comics in actual comic book form. Not only did that help clarify a lot of concepts, the very act of reading the book itself served to reinforce the ideas presented within. I would also recommend his follow up, Reinventing Comics, which looked at where comics are now and where the form could go.
