As member of a jury for admissions to a business school here in Paris, one of my tasks was to evaluate candidates' English levels. One student spoke to me so fluently that I expected him to say he had lived in the UK, or that one of his parents was Anglophone. Neither was true. In fact, during lycée in Paris, he was the lead singer in a band that sang almost exclusively songs by The Cranberries. He explained that since he didn't like singing lyrics he didn't understand, over the years, he consistently looked up words he didn't know. Bravo! Song lyrics were the exclusive source of his fluent English.
Let me add that I did observe that his English was quite colloquial, and so I suggested he supplement his vocabulary with something a bit more broad (that is, not to say "scholarly"). Any well-written newspaper, book, magazine or academic text would do. Find what you like, and do it in English. And keep using that dictionary!