Opting one's regional language as an optional subject in CSS is a good decision.Generally, Sindhi is more scoring subject.One can secure more than 80 or 90 marks.And it is, for often.I would like to suggest my Sindhi brothers to refer following sources in Sindhi:

1.Sindhi Adab Jee Mukhtaser Tareekh(core book) By: Memon Abdul Majeed Sindhi
2.Sindhi adab jo Mukhtaser jai zo By:Abdul Jabbar Junejo
3.Sindhi Adab Jee Mukhtaser Tareekh By: Akbar leghari
4.Kalachi (a quarterly magazine published by University of Karachi)
5.Ibrat Magazine.
6.Some old issues of Mehran(mag), Sindh(mag) Paigam(mag)
I hope the above mentioned books would help u a lot.