Before you begin, go through the previous years’ preliminary question papers & mains question papers and analyze them. This should give you a brief idea about the type of questions UPSC asks under each topic and the frequency of these questions. Knowing what’s important under each topic will make your approach easier.
To excel in any field one should have strong basics. The same applies to the Civil Services Examination. The basics here are the NCERTS from 6th standard to 12th standard. The following is the list of the NCERT books 
Note: Even if you strong in basics it is recommended to go through the NCERTS at least once to brush up your concepts.
Note: The books are available for download for free on the NCERT Official Website.

Class 6: Our Past I
Class 7: Our past II
Class 8: Our past III (Part 1 and 2)
Class 9: India and the Contemporary world – I and II
Class 10: Themes in World History, India and the contemporary world II
Class 11: Themes in World History (Industrial revolution)
Class 12: Themes in Indian History I, II and III

Indian Society
Class 11: Sociology: Understanding Society
Class 12: Indian society

Art & Culture
Class 11: An Introduction to Indian Art
Class 11: Living Craft Traditions of India (Chapters 9 & 10)
Class 12: Craft Traditions of India

Class 6: The Earth: Our Habitat
Class 7: Our Environment
Class 8: Resources and Development
Class 9: Contemporary India – I
Class 10: Contemporary India – II
Class 11: Fundamentals of Physical Geography
Class 11: India – Physical Environment
Class 12: Fundamentals of Human Geography
Class 12: India – People and Economy

Class 9 – 12
Class 9: Economics
Class 10: Understanding Economic Development
Class 11: Indian Economic Development
Class 12: Introductory Microeconomics
Class 12: Introductory Macroeconomics

Class 9 – 12
Class 9: Democratic Politics I
Class 10: Democratic Politics II
Class 11: Indian Constitution at work
Class 11: Political Theory
Class 12: Contemporary World Politics
Class 12: Politics in India since Independence

Class 12: Social change & development in India

Science and Technology
Class 7 – 10 (All the books); 
Class 11 and 12 (Selective)
Class 11: Chemistry (Unit 14) Biology (Units 4 & 5)
Class 12: Chemistry (Unit 16) Biology (Units 8,9 and 10)
Class 12: Ecology (Unit 10)

  • Chapter 13: Organisms & Population
  • Chapter 14: Ecosystem
  • Chapter 15: Biodiversity & Conservation
  • Chapter 16: Environmental Issues

Environment and Ecology
Class 12: Biology: Last four chapters (13 to 16) & Chapters on Biotechnology
After completing the above NCERTS, one should move to the standard books for the each subject. It is important that you study the books topic-wise (refer the syllabus), and not chapter-wise.
The following are the recommend books for each subject:

  • Previous Years Solved Question Papers Topic wise for preliminary Examination ( Any publication (Arihanth , Disha ....) )
  • Previous Years Solved Question Papers Topic wise for Main Examination    (Any publication (Arihanth , Disha ....) )

For Polity & Governance:

  • Indian Polity by Laxmikanth
  • Introduction To The Constitution Of India by D.D.Basu
  • Our Constitution by Subhash Kashyap
  • Governance in India by M Laxmikanth
  • SELECT CONSTITUTIONS  by Anup Chand Kapur, K K Misra

For Geography:

  • Certificate Physical and Human Geography by Goh Cheng Leong
  • Indian and World Geography by Majid Hussain

For Economy: 

  • Indian Economy  by Sanjiv Verma
  • Indian Economy: Performance and Policies by Uma Kapila

For History:
Ancient History:

  • India's Ancient Past  by R.S. Sharma
  • Economic Survey

Medieval History:

  • A History of Medieval India by Chandra Satish

Modern History:

  •  A Brief History of Modern India by Rajiv Ahir
  • India's Struggle For Independence by Bipin Chandra
  • From Plassey To Partition and After by Sekhar Bandyopadhyay

Post Independence:

  • India since Independence by Bipin Chandra
  • India After Gandhi The History Of The World’s Largest Democracy by Ramachandra Guha

World History:

  • History of the World by Arjun Dev
  • Mastering Modern World History (Palgrave Master Series) by Norman Lowe

For Environment:

  • Environment and Ecology - Biodiversity, Climate Change and Disaster Management by Majid Husain

For International Relations:

  • Challenge And Strategy : Rethinking India's Foreign by Rajiv Sikri
  • International Relations by PEU GHOSH

For Art & Culture:

  • Indian Art and Culture by Nitin Singhania
  • Facets of Indian Culture by Spectrum

For Ethics, Integrity & Aptitude:

  • Lexicon for Ethics, Integrity & Aptitude for IAS General Studies  Paper IV by Niraj Kumar
  • Ethics for Governance : Reinventing Public Services by B. P. Mathur
  • Ethics, Integrity and Aptitude for Civil Services Main Examination by Subba Rao

Preparation Stratergy: How to read & how many times to read
It is recommended that you read the Books at least 3 times.
1st Reading:
The first time you’re reading the books, read them like a novel. Highlight the important points using a highlighter (in case of eBook) or a pencil (hard copy). Keep a dictionary handy. If you come across any difficult word, look it up immediately. If you don’t understand a specific concept, use Google and read up about it.
Note on highlighting (how to know what to highlight):
It’s important that you know what information to highlight and what to ignore. This can be done by analyzing the previous year question papers. Analyzing previous years’ question papers will help you understand the type of questions asked under each topic. For instance, if you’re studying about the various Government schemes and policies, you should focus on advantages & disadvantages of the policy, need for that policy and its approach. If you’re studying about a specific industry, your focus should be on the location of the industry, the factors affecting its location, challenges faced, opportunities and its current situation.

2nd Reading:
Give a gap of about 2 to 3 days after your first reading, before you begin your second reading. During your 2nd reading, ignore the unwanted details and focus on the notes you had highlighted during your 1st reading. It’s a good idea to try and recall whatever you learnt during your 1st reading before revising each chapter.
3rd Reading:
Now that you’re done with your second reading, it’s time to summarize what you have studied. The 3rdreading should be relatively faster than the 1st and 2nd. During the 3rd reading glance through each chapter’s highlighted parts. Try and relate the concepts you’re studying to current affairs, and connect the dots.
You can follow the above given approach or use it as a guideline to formulate your own approach and strategy for your preparation.