Understanding Comics is both an excellent treatise on comics and a working example of the form. Scott McCloud explains the medium within the medium–highlighting one of the unique strengths that comics have.
McCloud makes the argument that comics fill the gap on the scale that has purely representational images on one end (visual ‘high art’), and on the other end, the realm of purely arbitrary images (aka words, as ‘literature’). But the comics niche has been trivialized as a mere diversion of pop culture, and that ain’t right. (See Highbrow/Lowbrow for similar cultural divisions and how they came about)
McCloud traces the roots of comics back to the early days of literacy, before literature and art went their separate ways. Drawing on this union is where comics set themselves apart as a unique form of visual communication. I see a parallel here with Beautiful Evidence (my review), where Tufte has a whole chapter called “Words, Numbers, Images, Together.” Those were the good ol’ days when words and doodles got along just fine without ridicule.
After the history, there’s an extended analysis of form and style and structural elements.
It’s interesting to see McCloud use an argument that is revived in Steven Johnson’s Everything Good is Bad for You (my review). Namely, that comics are more demanding of the reader. The storyline isn’t completely spelled out for you. The selected elements of the story are presented together, but you have to fill in the gaps between frozen moments in time, to give them life. As McCloud says, the comics reader becomes a participant.
Though it is probably beyond the scope of the work, I’d only ding McCloud for not going into enough depth. I’m sure there would be some copyright issues (grrr!), but I wish he were able to do a longer work with more case studies and analysis of the form. That task, however, is left for the newly-educated participant-reader. Which is perhaps how it should be.
Mark Larson 
[…] Reinventing Comics is the middle child in the McCloud comics trilogy. I found it to be the weakest and least interesting of the three. (see my reviews of Making Comics and Understanding Comics) Not bad, but nothing special. McCloud himself sums up nicely: I believe that Reinventing Comics has genuine flaws. The two halves don’t always work well together, the storytelling is frequently stiffer and less convincing, and my enthusiastic advocacy of online comics is rarely tempered by some of the bleaker, more pessimistic scenarios offered by other writers in recent years. It was a harder book to write than Understanding Comics and, from all reports, a harder book to read. […]