At Blambot, Nick Piekos has a very informative piece on the conventions of comic book lettering called "Comics Grammar & Tradition." Like the Comics Lettering Tutorial in the previous post, this comes from a working letterer and so it really gets into conventions and theory behind the storytelling choices. For example, Piekos explains that the balloons to the left are called burst balloons:
Burst Balloons are used when someone is screaming their dialogue. They tend to be more irregular and chaotic than the radio balloon, perhaps with a heavier stroke. Burst balloons typically aren't italicized, but are often bold with certain words enlarged or underlined for even more emphasis. A less punchy variation on the burst balloon is a regular balloon with a small burst where the tail meets the balloon.
Along with all of the tips for comics creators and letterers, there's a lot of extremely useful material for those of us who want to think about how comics work as a medium. It's definitely worth a look.