I think it says quite a bit that I had already been thinking of Outside In before it was even mentioned in this article. Outside In truly is one of the great works of math communication.
I'd argue that its format, a dialog between a novice who is yet still sharp (is able to be the one that makes the connections between ideas) and an expert who guides with pointed questions (fills in background context that is needed for the problem at hand), is one of the most effective ways of formatting mathematical communication. Surreal Numbers by Donald Knuth is another great example of the style I'm referring to.
I'd argue that its format, a dialog between a novice who is yet still sharp (is able to be the one that makes the connections between ideas) and an expert who guides with pointed questions (fills in background context that is needed for the problem at hand), is one of the most effective ways of formatting mathematical communication. Surreal Numbers by Donald Knuth is another great example of the style I'm referring to.