Holy Toledo! This book was fantastic. The now-widespread idea that comics are capable of deep expression and not just a low form of entertainment comes from 'Understanding Comics'. The format has come a long way in 25 years, but McCloud's work is still an essential read for any comics, graphic novel, sequential art fan. He breaks the medium down and explains the mechanics of its storytelling capabilities frame by frame.
I never studied art theory, so many of the concepts here were completely new to me. It was informative and reading this felt like having a personal lesson. That may not sound like its for everyone, but it should be. Comics are simple medium for a reader to understand, but only when its done correctly and McCloud offers a great deal of advice on how to go about creating your own comics and makes no rules that he doesn't encourage a new artist to break.
There's a lot of bombast and bright, expressive earnestness and hope which invites eyerolling and (legitimate) complaints, but I don't care. I enjoyed the hell out of it.