[Describing the technical workings of the Atari 2600][link], it is not an emulator, or even really a “how to,” but more like a… Documentary App. I love this kind of thing.
It is Volume One in what will become a series, and if you don’t buy this you are an asshole.
[link]: http://itunes.apple.com/us/app/2600-magic/id351446523?mt=8# “2600 Magic for iPhone, iPod touch, and iPad on the iTunes App Store”