I really like Strunk and White's Elements of Style, because it's not only a classic in terms of grammar, it's also just a somewhat amusing, well-written book. Beyond that, if your top priority is improving your writing in general, Garner's Modern American Usage is a nice one; it's one of those books that writers refer to year after year when trying to figure out tricky usage issues.

Garner's book is very complex. I'm not sure how will I read it. Thanks for elements of style. I remember this book was recommended by G. Mankiw. Again thanks. Does collection of essays be helpful? May be there is a specific approach in reading good essays that can help improve?

Reading things that are well-written is always helpful in grasping the general tone, grammatical structures, and idiom usage of good writing. If you're specifically aiming at improving writing in a way that will be helpful in gaining entry to a U.S. business school, make an effort to find modern American writing; older writing can contain antiquated usage. Even some periodicals can help in this; examples that come to mind immediately that I personally enjoy are publications like The Atlantic Monthly and The New Yorker. Avoid anything that's written in an overly casual or colloquial tone, since it's more likely that those pieces will contain the kind of "accepted" grammatical errors that can cause trouble on the GMAT. Essays on business or current events have the added benefit of possibly providing material for your AWA essays, since they can give you good examples to use in supporting your conclusions.