The eightieth comic: #634 Understanding Comics
I'm not sure what the best point would be to to read this, but I feel like 80 comics in, I got a decent base to understand the specific comics and styles Scott McCloud was referencing throughout - knowing French comics, for example, is incredibly helpful.
This work starts off by defining what a comic is, then after that analysing and explaining the medium in several ways - levels of abstraction, words versus pictures, as well as exploring the gap between panels. It's an engaging thesis - mostly right, and leaving plenty open to resolve - that gives me a lot more to think about when trying to understand comics in the future. The differences between different styles - especially Japanese and Western - is well explained here and it feels like subsequent works really made use of these ideas to create a way to produce comics.
This means that this is not only a fascinating read - and it really is worth reading for any fans, or anyone trying to understand comics and why they qualify as art - but also give you a better grounding for other styles.