Groensteen is the Dean of francophone comics studies and with good reason. His analytical exploration of how comics work is deep, thorough and extremely well-thought-through. The translation is a little clunky (thereby knocking a point off), but don’t let that put you off if you’re serious about comics
One of the seminal works of comics theory in the English language, and the fact that it’s a comic itself just adds to the awesomeness. There’s faults with it to be sure, but as a starting point to the wonderful world of comics, you can’t go wrong.