I've mentioned Scott McCloud once before. This is his book, Understanding Comics, which is pretty much a must-read for anyone serious about creating comics. His second book, Reinventing Comics went on to discuss comics in the new medium of the World Wide Web (gosh). Although some of his conclusions are much-debated, it's difficult to claim any sort of deeply informed insight into the world of webcomics without having read the books. And even if all you do is read comics, it's worthwhile taking the time to read McCloud.