@For those candidates who’ve chosen “Hindi” as their Compulsory Indian language paper.
In 2010′s Mains exam, 318 candidates failed in this paper. (according to UPSC reply in RTI)
Now the question asked by many, which book to refer for Compulsory Indian Language Paper (Hindi)
Since I’ve no idea about the book (in my case it was Gujarati), so I asked Prabhakar Ranjan (AIR 339/CSE 2011) in the interview, and according to him, a good book is “Samanya Hindi (Unique Publishers)”