I just finished Wolk's Reading Comics and I highly recommend it. The first half of the book describes comics in terms of the medium, the readers, the authors, the subjects (superheros vs everything else!), and the visual styles. The last half of the book contains reviews of works, and in reflection on the first half of the book, Wolk does a great job of drawing one into the aesthetics and tactics of comics. A very enjoyable read especially in conjunction with field trips to a good comics store like Incredible Pulp in Narragansett.