@Azmatullah
yeah,bro u can opt psychology,its an interesting and well scoring subject,books are many times mentioned b4,but still i would recommend Myers ,and Feldman;sand Zimbardo's books,as it will b in your knowledge that not a single book can cover the whole syllabus for CSS exam so one must take help from more than one book,furthermore you can buy Sherbano's Psychology book usually taught at graduation level,it will give you main idea and furthet elaboration you can obtain from the books mentioned above.
still ,if u find any difficulty you may post your queries here,v all are here to help you out.
wish u luck
thanxxxx