Vstudy books are from a Korean company - http://www.vstudy.co.kr
There are some 7-8 volumes of these books. I have only volume 1, that too without the answers. I like the book. Infact I would rate it second only to OG; not for the concepts, but for the questions in the practice tests.
Also, rumours are that these questions are from actual GMAT tests
Anyone who has never made a mistake has never tried anything new. -Albert Einstein.