We've covered a lot of things in this book and, from here, you'll need to go somewhere and expand your knowledge. In practice, you'll become a lot better at doing your job.
So where do you go from here?
The best approach from here is more reading, some more reading, and then some more reading!
I've learned that the more you read, the more you know, but you also realize how little you know. That shouldn't stop you from learning and becoming as good as you possibly can.
One of the things that help us become better is exploring, learning, and always trying out new things. Take responsive frameworks such as Bootstrap or Foundation for a spin, see how they work. You'll learn a lot by investigating other people's code. If you understand how things work, and work well, this will guide you in developing your own custom solutions. The best lessons learned are those that are learned while making mistakes yourself, or learning from others' mistakes, which can often save a lot of time.