While reading books will definitely help to some degree, I think really only practicing speaking and writing will really develop that part, because reading is more passive? You can absorb and know the words you're reading, but they still won't be there when you're reaching for words while writing/speaking. Even with my degree of fluency in Japanese, I am much slower writing in it than in English, because I just don't use that part of it as much. So while I can easily reand and understand a novel 100%, I still feel nervous about writing in Japanese and obsessively recheck stuff again and again before sending it out.