Let’s Make a Map by Mike Bostock.
From the post:
In this tutorial, I’ll cover how to make a modest map from scratch using D3 and TopoJSON. I’ll show you a few places where you can find free geographic data online, and how to convert it into a format that is both efficient and convenient for display. I won’t cover thematic mapping, but the map we’ll make includes labels for populated places and you can extend this technique to geographic visualizations such as graduated symbol maps and choropleths.
Excellent introduction!