What Do You Mean It's Not for Kids?: Understanding Comics, Making Comics and Reinventing Comics each have a cartoony, kid-friendly art-style, but all 3 books tackle complex ideas about artistic theory that most people below a certain age would likely find boring and/or overwhelming. Oh, and Understanding Comics has a random picture of a naked clown lady.
Much like comedian Bob Saget's stand-up work was more mature than his televised work, McCloud's online comics are noticeably more adult than his printed comics. People only familiar with the latter might be very shocked upon their first visit to his site.