Well whatever Anirudh said is totally valid, but I understand its not as easy as it sounds to follow these things. Even I suffered from the same in my earlier days. Word Power Made Easy by Norman Lewis is brilliant book that helps you build a strong vocab. I would suggest you to buy this book and keep reading it again.
The book does not forces you to memorize any word, it explains the origin and history behind the word and hence making it easier to remember the words.