Arihant has launched a "Arihant Prep" app. While the full book isn't free, they often sell (i.e., buy only the Vocabulary section for ₹99). This is ideal if you only need a specific part of the syllabus.
SP Bakshi is highly regarded for its structured approach to the English language. Unlike generic grammar books, it is specifically tailored to the changing patterns of modern competitive tests. Objective General English - SP Bakshi - Google Books sp bakshi english book pdf google drive