Word Power Made Easy is the best book to improve your English Vocabulary especially for people who are preparing for the GRE. Roots, pronunciation, spelling, etc. are best explained in this book. It also has practice exercises in the end of each session so it is recommended to have a pen/pencil with you. Don't mind the thickness of the book, it has extra space for practice which is for your own good.
DO CHECK THE PAGES OF THE BOOK AS SOON AS YOU RECEIVE IT. I received one with about 20 blank pages.