Mere Mortals This is simply one of the best and most detailed introductions to D3 that I have seen. Seriously, if you want to have a chance of building your own custom visualizations and not just hacking at someone else's prebuilt example, read this article!
D3 Tutorial Set Scott Murray offers up a great set of tutorials that walk you through a lot of the basics.
Data for D3 As Jerome Cukier says, "[p]utting your data in the right form is crucial to have concise code that runs fast and is easy to read" and this article discusses and shows different ways you can format data for use with D3. A highly recommended read!
Setting up your Development Environment This is another excellent tutorial by Jerome Cukier focused on getting the development environment up and running (a crucial and often frustrating step).
D3 Workshop Slides by Mike Bostock The following set of slides presents a "large swath of introductory material" that was initially given at the VIZBI 2012 conference on March 5th over 3 hours.
D3: Data-Driven Documents This paper is the paper for citing D3 and describes a lot of the design decisions made and the rationale behind them.