I think there is a third one, also.
Shows you how out of touch I am. Heh.
The third one is called 'Reinventing Comics' and I hear it's quite controversial in the comics world, mainly because it argues against everything the big two (Marvel and DC) are doing at the moment. To be honest, even though I love his other two books, I'm not sure about the third one, I've never wanted to read it just because the idea of one guy, even if he IS a genius, saying 'you can revolutionise comics if you all do what I say!' irks me somewhat.
I am a fan of Scott McCloud, but at the same time, you get hints of this little edge of egotism to him, as though he thinks he knows best about everything, and the reason his comics aren't that popular ('Zot!' won awards or something I hear, but I'd never heard of it until I read 'Understanding Comics') is because not enough people are as smart as he is. I can appreciate when he's teaching stuff, and I think that's great, I just like it less when he gets all preachy and 'you should be more like me'.