McCloud begins by defining comics ("juxtaposed pictorial and other images in deliberate sequence"—"sequential art" for short) and then uses his definition to explore the history of the medium, showing that comics have really been around a lot longer than comic books. Then he starts going into how comics work, primarily focusing on the visual aspect and examining how a reader responds to certain images and icons. He makes a really interesting observation about the degree of reader identification possible in different art styles: a cartoony drawing without much detail allows the reader to more easily see himself as the character, whereas a more realistic drawing doesn't leave as much room for interpretation. It's not something we do consciously, but a lot of McCloud's observations are about our subconscious reactions that inform our readings. He develops a whole system of aesthetics for the pictorial vocabulary, a triangle with vertices of Reality (where pictures represent reality), Language (where pictures simply communicate an idea), and the Picture Plane (where pictures are just shapes).
Then he moves on to look at comics themselves and makes a point I never really thought about before: we fill in the spaces between panels. What is on the panels is what is on the panels, but our minds are what do the work of connecting them. This leads to a look at the wonkiness of time in comics and the ways in which panels can fit into the time-space continuum. He talks about the use of lines, the different ways in which dialogue/words interact with the pictures to tell a story, and color. And he has a whole chapter about the creative process. Throughout the book, he compares the evolution of comics in the Western world to that in Japan, where comics/manga are quite different from what we're used to.
It would be useless to try to summarize the whole book when you can just read it yourself; it's a pretty quick read. I highly recommend it to anyone interested in comics, as it does make you look at them in a new light, not just because McCloud staunchly defends comics as an art form but because he makes you think about what you're actually doing when you read and process a comic.
And, armed with this knowledge, I can give you my (generally non-spoilery) assessment of Buffy: Season 8 so far!
The Long Way Home (Joss Whedon): B- (I was pretty underwhelmed by the first arc. Joss seemed like he was trying way too hard to recapture the characters' voices, and the whole thing seemed like it was just designed to please fanboys, with three issues in a row using the Appearance-of-a-Character Cliffhanger. There wasn't a whole lot of substance.)
"The Chain" (Joss Whedon): A (This issue was fantastic. It's better written and more emotionally compelling than all four preceding issues combined, and it's about a character we've never seen before. Hands down the best thing to come out of this idea so far. It would be an A+ if it weren't for the random faeries. What the hell?)
No Future for You (Brian K. Vaughan): B+ (I had really high expectations for this arc, and while I thought Vaughan did good with the character, I don't feel like we really learned much more about her. It was a good story, though.)
"Anywhere but Here" (Joss Whedon): B (I found this issue really confusing, and I'm still not sure I understand what the hell it's about and what happened at the end.)
"A Beautiful Sunset" (Joss Whedon): A- (I think Joss is at his best when he's delving into the thoughts of the characters, so an issue of Buffy musings works much better than...whatever the hell the last issue was. And it's always good to really examine the consequences of what Buffy did in "Chosen," as he did in "The Chain." Plus, the little tease with the identity of Twilight was cute. Even when individual stories haven't grabbed me, I'm still caught up in the ongoing/seasonal arc.)
Wolves at the Gate (Drew Goddard): B+ (I really enjoyed the humor in this arc, even though there was a serious story running through it. Ultimate Drew kept it pretty light and also made Dawn pay off in a big way. Pun possibly intended. More importantly, he did well with the various character relationships.)
OVERALL: B/B+ (I gave the four-issue arcs four times the weight and did the math. So far, Joss is bringing the series down more than he's bringing it up, though a lot of the damage was in the first arc. The next arc looks exciting, and I think the series has found its footing.)
GEORGES JEANTY'S ART: C+ (Oh my God, it's kind of awful. I've been spoiled by artists who can actually draw people and bother to draw faces on everyone in every panel. Characters can look different from panel to panel, and you can't always tell who a character is just at a glance. Geez, Eduardo Risso's art may be (intentionally) ugly as shit, but at least it's comprehensible.)
JO CHEN'S COVERS: A++++++ (Holy God, so beautiful. So hot. Painted covers FTW, always and forever.)