if you already are learning D3 and want to learn more, I'd say just look at a lot of examples, study them, and experiment with its code. There are lots of examples here on the D3js Wiki https://github.com/d3/d3/wiki/Gallery
. In looking at examples, you will be question driven as to what certain notations are or methods unknown to you, which then you'll then have to find those answers. That journey in searching for directed answers and experimenting with code yourself will help you better understand D3. My 2 cents.