Hello,

When it comes to preparation for UPSC,you have to make  your foundation strong which you can do with NCERTs , so go  through these first,as you progress, move on to other relevant books as mentioned below;

For Prelims, refer the following books:

History ;

NCERT Class XI and XII
India's Struggle for Independence by Bipin Chandra
For Indian Culture, refer art and culture by Nitin Singhania

Geography ;

NCERT Class VI to XII
GC Leong's Physical Geography
World Atlas

Economics ;

NCERT Class XI
Indian Economy by Ramesh Singh

International Relations & Polity ;

NCERT XII Political Science with current affairs
Indian Polity by Laxmikanth

CSAT ;

Verbal and Non-Verbal Reasoning by RS Agarwal
M Tyra's Quicker Maths

Coming to Mains Exam, you can refer the following books for four General Studies Paper;

GS I - For History , Class XI NCERT book with India's struggle for Independence by Bipin Chandra,India after Independence by Bipan Chandra,History of Medieval India by Satish Chandra

For Geography refer World Geography and Geography of India by Majid Hussain, GC Leong's Physical and Human Geography with NCERT Class XI book.

For GS II - Polity and International Relations , refer Indian Polity by Lamikanth,India's Foreign Policy by Rajiv Sikri

For GS III - For Economics refer Indian Economy by Ramesh Singh, read ARC report along with Economic Survey and Fiscal Budget.

For GS IV - For Ethics refer a book named Lexicon by Niraj Kumar, also you can go through study material of IGNOU.

Remember to keep resources minimal always, read one book many a times instead of many a book one time, revision is the key here.

In addition to books, take a print out of the syllabus and learn it by heart, through this you'll get to know what to read and skip.

Start reading newspaper on a daily basis, the best option is The Hindu or The Indian Express, put more emphasis on editorials part, if necessary make notes.

Practicing answer writing in advance, even in a little way, is must, writing mains is huge task, it needs practice, practice and practice to complete appropriate answers in three hours,there are many sites which provide questions on daily basis you can avail that. Try to give a holistic viewpoint while answer writing by providing political,economical and social aspect. You can also check out topper's answer sheet available in websites in order to have an idea about how to present your answer and fetch more marks.

Practice previous year question paper available in UPSC website,go through mock papers, and test series on regular basis prior to exam.

I will enlist down some of the top most referred books for UPSC entrance exam preparation -

1. For Geography, use NCERT (old version books) and for new er syllabus use G C Leong World Atlas
2. Indian Art & Culture subject by Nitin Singhania
3. For Polity, refer book by M Laxmikanth
4. For Economoics - Ramesh Singh, Jain and Ohri
5. Environment - Majid Hussain
6. Keep reading newspapers everyday to know about all the current affairs.

Refer to the website below for more detailed information about the same..

https://competition.careers360.com/articles/books-for-upsc-ias

Best wishes. Thank you.