You require the NCERT books, depending upon the optional subject you choose.
Some of the books recommended to take up self study for IAS preparation are:

• History Of Modern India  Bipan Chandra. (History)
• Indias Struggle For Independence  Bipan Chandra. (History)
• India's Ancient Past  R.S. Sharma. (History)
• History Of Medieval India  Satish Chandra. (History)
• The Wonder That Was India  A.L. Bhasham. (Culture)
• Indian Art and Culture  Nitin Singhania. (Culture)
• Geography of India  Majid Husain. (Geography)
• Oxford School Atlas  Oxford. (Geography)
• Certificate Physical and Human Geography  Goh Cheng Leong. (Geography)
• Indian Polity for Civil Services Examinations  M. Laxmikanth. (Polity)
• Indian Economy  Ramesh Singh. (Economy)

Apart from books there are recommended magazines as well:

• Yojana
• Economic and Political Weekly