If you have done the study of New Concept English II, go with this book directly. 1200 key words in this book, it may double, or triple your vocabulary step by step, and much stronger in using them:
1, 250 roots, 4keys words under each root, very well explaning and how to use. It is also a thread for you to look up more valueble words in dictionary, probable 10-15words more.
2, Greek/Latin phrases borrowed into English, using them makes your English more tertiary.