Scott McCloud's books are pretty much required reading for anybody interested in comic books, whether you read them or create them. He started off with Understanding Comics, expanded his vision in Reinventing Comics, and then capped it all off with his most recent work, a fabulous treatise on how to create comics and graphic novels, Making Comics. I don't make a lot of "must have" recommendations, but I would say that if you enjoy comics, or want to make your own comics, you really should own all of these. Don't borrow them, buy them. They're worth having at hand.
This week, McCloud finally released his promised online "Chapter 5 1/2" for Making Comics, which specifically deals with webcomics and the unique needs and processes that this form deals with. Well worth a read.