When you talk about important comics, Watchmen usually gets mentioned, but From Hell (also by Alan Moore) is probably the better book. It’s a Jack the Ripper story in 14 chapters, but it’s really about the birth of the 20th century (which makes it a bit more significant historically than rewriting the superhero genre).
I remember borrowing a buddy’s copy years ago, and not knowing much about Jack the Ripper beforehand I came out a little underwhelmed. On my re-read I think there were two reasons for that. 1) I was concerned mainly for who did the killing and the mundane mystery of it all, which the book pretty much dissipates early. I felt like I was missing something, and I was: the significance of everything that was going on. 2) I tried reading the appendices all in one shot after finishing the story. This time I read each chapter’s notes directly after finishing the chapter and it worked so much better. I also could linger more (since I didn’t have my whole “what happens next?” portion of my brain dragging me through) and appreciate all the interesting things they did with time (which is where it’s easiest to tell this is the same guy as the writer of Watchmen).
Anyway. This time I let the chapters seep into my brain where they could slosh around with everything else and they’re doing much more good this time around. Though really, it’s the appendices that make the book. As I am not a Ripperologist I love that feeling that Mr. Moore is sifting through all the detritus and pulling out the most interesting bits and forming it into how he thinks the story may have gone. And then demolishes the entire idea of there being any way to talk about anything having one solution.