If you are an absolute beginner, you may refer to the following booklist and material sources (for both pre and mains as I think the preparation of the two cannot be seperated) -
1. Indian Polity - by M Laxmikanth, Tata McGraw Hill Publication
2. Geography - NCERT Old 11th and 12th class (Photocopy available in bookshops in Delhi and other places in other cities at popular preparation destinations etc). Also available on internet, search on google. You may also go through new NCERTs for 11th and 12th, they are also good.
3. History - Bipan Chandra, "History of Modern India" Orient Black Swan Publication
4. "Indian Culture and Heritage" - National Institute of Open Schooling. Also available on internet on website of NIOS, search on google. Also refer to the Art & Culture :NCERT Class XI – An Introduction to Indian Art; Art & Culture
5. Economy - Ramesh Singh, Tata McGraw Hill Publication or any other standard book like Dutt and Sundaram of S Chand publications.
6. Environment and Ecology - any popular coaching institute notes - (Photocopy available in bookshops in Delhi and other places in other cities at popular preparation destinations etc)
7. Science and Technology - any popular coaching institute notes - (Photocopy available in bookshops in Delhi and other places in other cities at popular preparation destinations etc)
8. Magazine - Frontline (if you have time, else skip it)
9. Newspaper - The Hindu or Indian Express (read only one, it would be sufficient)
10. World History - Old NCERT Books of History 9th and 10th class, new NCERTs are more illustrative and colorful, you may also go through them as well. (for Mains paper only)
11. Ethics Paper - try to note down from newspapers, books etc examples of ethical conducts etc. Try to think on your own. It is more of a paper of reasoned thought and rational arguments. Don't read too much of material, just practice a few case studies. Use your original thoughts.
12. India - An annual book published by Publication Division of Govt of India for overview about government scheme and working of government of India
13. CSAT - Take notes of any popular institute or some standard book on reasoning and aptitude.
14. India After Independence - Bipan Chandra, Penguin Publication (for post -independence history, for Mains paper only)
15. Past years solved papers of both GS Preliminary and Mains. Read them again and again so that you have an idea about the type of questions they ask.
16. Download syllabus of both pre and mains and read it many times so that whenever you read newspaper, you know that what has to be read and what has to be left. Don't read multiple books on same subject, but read single book or notes multiple times. Material with you should be only so much only that it is manageable at the time of exam.
It is advised that limited books and material is refereed as the exam is not only about reading so many books, but about managing what you have read. It is not so important that how many books on a topic that you read, but how well you read them.
Any suggestions or queries can be sent in form of comments for the benefit of all.
Best of luck!