Whether it's fiction or non-fiction, you increase your understanding of words by reading as much as you can provided it gives you a challenge while reading. If you are using the knowledge to increase your vocabulary for a graduate exam, then you can read medical journals, newspapers, or other content that focuses on the subject matter in which you want to major.

When you stumble upon a word that you're not quite sure of the meaning, don't just skip over it or guess its proper usage. Get the dictionary you carry in your pocket or use at your desk and look it up. Get a feel for the word and how it can be used in other sentences. You can even use it as a new word to practice throughout the day. Give yourself time to focus on the new vocabulary word and even go back to the original passage of the book to make sure the meaning is clear to you.