Apologies to Scott McCloud, natch. I love Zot and his Comics series, but come on, you don’t have to reach so much in order to fill up page space.
But seriously: Understanding Comics as a must-have, and if you want to make comics, get Making Comics (!!??!!). It is very thorough and very enjoyable, especially if you’re a total geek like me. (Get Reinventing comics if you want to be a completionist)