When you reach a certain level in your target language, you can start to read books without having to look up every other word. How soon you reach this point obviously depends on what book you want to read, and you can just as well start with children's books as with unabridged novels, depending on your current level. Simply pick a book that you think will interest you, no matter what it is about, and start reading it. It is a sure way to increase your vocabulary quickly.