Times have changed. 5 years ago I had to struggle to find a good list of books recommended for any optional for UPSC CSE on internet. Today you can get several websites dedicated to help out a beginner for UPSC and most of them can give a comprehensive list of books required for almost any optional. However in their efforts to be exhaustive, they tend to include more books than required leading to wastage of time as well as scaring off someone who is trying to get feel of the study required for UPSC. I am putting down my own list of books, starting with GS because it has no defined syllabus.
General Studies Booklist:
Polity - Laxmikanth (both prelims and mains, nothing more nothing less)
Indian History ( Ancient and medieval) - Vajiram's Yellow Books or sections from Unique Guide (Prelims only)
Indian National Movement - Unique Guide (Prelims), Spectrum( Mains), Bipin Chandra ( for those who have time and want a DEEP understanding)
Geography - Unique Guide (Prelims, Both India and World Geog.), NCERT Class XI-XII & vajiram's Yellow Book(Mains)
Indian Economy - Vajiram yellow Book (Prelims), Vajiram's Class Notes( Mains), Pratiyogita Darpan Special Issue ( Prelims and Mains)
Science & Technolgy - Vajiram's Class notes, Tables of chemical names, etc. from Unique Guide(Prelims), TMH Sci. & Tech. ( Mains)
Bilateral and International relations - Vajiram's Yellow Book, World Focus magazine
Current Affairs - Chronicle Magazine, The Hindu newpaper, Wizard's Special issue on Current Affairs (Prelims, comes out in March)
Statistics - Spectrum ( Mains)
Caution: - 1) I could never study Ancient & Medieval History. Hence ask someone knowledgable for these topics.
2) Do not study any other book except Laxmikanth in Polity. It is quite lucid and exhaustive.
3) Spend atleast 1 hour everyday on Hindu.With changing pattern of GS wherein it is becoming more and more random and analysis-oriented, The Hindu is the only thing going to save you. Make short of notes of articles on burning issues. In order to streamline your study of The Hindu, see the headings under which articles are written in Chronicle. For example, Bilateral, International, nationa issues, Environment, etc. While readig Hindu choose articles that fall under these headings and read them.
Do not keep cuttings from newspaper, study it then and there and make notes if necessary. You will never ever study the newspaper cuttings. Do not bluff yourself.
4)Anything else other than mentioned above, do not study anything else. According to me it would be a waste of time.
Most important part is to think. Try to have a opinion on any issue you come across. You must think of points both in favour and against the topic. Your thinkig faculty would be your only defence against the vagaries of UPSC.
Booklist of Sociology and PubAd would follow later.