Because English is not your native tongue, you are definitely taking the right approach of improving your understanding of advanced English. At the same time, indiscriminately reading as many books as possible will overwhelm you, and won’t necessarily translate to a strong GMAT score. So you will want to target your reading following the guidelines below.
1. Read Challenging Pieces and Construct Meaning
One of the keys to success on the RC section is being able to digest a dense amount of information in a short period of time. Reading The Economist, The Wall Street Journal and Scientific American can be helpful, but you want to make sure that not only are the articles difficult, but also that you are able to intelligently discuss whatever you are reading.
For instance, if you are in the middle of a long article and someone asks you what you are reading, you want to be able to offer a sophisticated summary of what you are reading. This skill is essential when dealing with GMAT passages so you want to make sure you can do the same when reading articles that will likely not be as difficult as those found on the GMAT.
2. Pay Attention to Idiomatic Expression
By reading you will be exposed to the way that English is used in an academic context. Many of the dreaded idioms on the GMAT come up all the time in article in the sources quoted below. Listening to news on-line (BBC and CNN) will also allow you to hear formal English (most sitcoms – whether British or American – will not help you much with the GMAT). Developing your ear this way can make learning GMAT idioms much easier. I wouldn’t worry too much about memorizing the meaning of words/vocabulary. Of course have a dictionary at hand so you can make sure that you are learning words that are crucial to understanding the meaning of a text.
3. Study Grammar Basics
Princeton Review and Kaplan both offer grammar primers. Though their focus isn’t explicitly the GMAT (of course they have these guides as well) improving your understanding of English grammar basics is essential for when you have to attack the more difficult stuff (the
MGMAT is then a great guide at this point).
4. Never stray to far from the GMAT
18 months is a long time, so you do not want to immerse yourself in formal English at the expense of ignoring the test itself. Practicing verbal questions, preferably easier ones, will give you exposure to your test and, in some cases, allow you to apply some of the knowledge you’ve gathered from reading articles and studying grammar books.
Good luck!