This book - Word Power Made Easy - is according to me to best way to learn english and improve your vocabulary.
I would reccomend this to students who are going to appear for exams in which English language plays a vital role in scoring.
This books by Norman Lewis offers learning words with the help of. root words of a word and its origin ( termed as etymology in this book) .
I myself, am preparing for GATE exam. My friends who are preparing for GRE, IAS also refer this book.
Also, this book is useful if you just want to improve your command over english language, as we Indians have to stuggle a lot to speak and understand English especially if one is not fron English household.
Overall, this is, according to me the best book to improve your English Vocabulary, Comprehension skills, speaking and social skills. If your are thinking of getting a copy, don't think, just go for it.
