The same questions as some years back: How to get started with it ?
The good news: There is plenty of material out there about D3, books, websites, tutorials. I purchased a couple of books from Packt Books and O’Reilly as PDF version, I think they have the most books, though most of them based on earlier D3 versions.
I wont recommend a specific one, most follow the same principles, a short intro to the DOM, explain the D3 basics followed by more or less complex samples. As with any other technology or programming language you learn most hands-on, there so many examples and good tutorials available. I never attempted to understand everything, understand the concepts, dissect existing visualizations and get creative !