Scott McCloud's Understanding Comics: The Invisible Art
I'm dropping this book back to the library tomorrow. There is no excuse not to reserve it.
To my knowledge, art appreciation classes never cover comic books. Most literature professors omit them from the curriculum (exception being P. Andrew Miller with his "Studies in Literature: The Graphic Novel" course). One of the chief reasons is that comics don't fall into the basic theories and understanding that we have of visual or written art.
McCloud provides criteria involving sound impressions, projection of time, sequences of action, the gutter (the white space between panels), color, and style. Many of his main points about comic art also apply to other art forms (he provides a six-tiered system of artistry that explains the difference between artists like Joyce, Picasso, and Orson Welles from Dickens, Charles Schulz, and Frank Capra). In a shameless plug, I dare anyone to read that section and not think of Nathan Singer.
One practical benefit I gained from the book is that I can now understand the appeal of the Japanese comic style known as masking in which features a very cartoonish character against a highly detailed realistic background. I'm still not a big fan of the style but I can understand psychologically what's going on.
I doubt if comics will ever gain real critical attention but McCloud at least establishes a framework to judge and evaluate comics as an art.