Choosing an optional subject is the most crucial part of the UPSC civil services preparation as the optional subject is the determinant factor of one's selection. One should choose the subject according to the educational background or interest. If someone is a science graduate but finds geography an interesting subject rather than chemistry or physics then it is better to opt for geography as an optional subject. The most important thing is to limit the materials and revise more, above all strategising is of paramount importance. Below is the list of recommended books for various optional subjects
HISTORY.
History of modern India – Bipin Chandra.
India since independence – Bipin Chandra.
Medieval India: From Sultanate to Mughals, part 1 – Satish Chandra.
Medieval India: From Sultanate to Mughals, part 2 – Satish Chandra.
India’s struggle for independence – Bipin Chandra.
The Wonder That Was India – A.L Basham.
Ashoka and Decline of Mauryas – Romila Thapar.
India’s Ancient Past - R.S. Sharma
Historical Atlas of India.
Mastering Modern World History – Norman Lowe.
History of the World – Arjun Dev.
GEOGRAPHY.
Physical Geography – Majid HUssain.
Certificate Physical and Human Geography – G.C. Leong.
Physical Geography Made Simple – Rupa Publications.
Dictionary of Physical Geography – Penguin.
Evolution of Geographical Thought – Majid Hussain.
Economic and Social Geography made simple – Rupa Publications.
Models in Geography – Majid Hussain.
Dictionary of Human Geography.
Oxford Students Atlas.
India – A Comprehensive Geography – Khullar
Regional Planning in India – Mahesh Chand.
Public Administration.
Administrative Thinkers – Prasad and Prasad.
Indian Public Administration – Arora and Goyal.
New Horizons of Public Administration – Mohit Bhattacharya
Public Administration – Fadia and Fadia
Public Administration in India 1st Edition – Maheshwari
Public Administration in India – B.L. Fadia
Public Administration – Laxmikant.
Public Administration and Public Affairs – Nicholas Henry.
Essentials Of Organisational Behaviour – Robbins Sanghi and Judge.
Pub AD a New Look – Mohit Bhattacharya.
Public Administration: Concepts and Theories – Basu Rumki.
Sociology.
Sociology Themes and Perspectives – Michael Haralambos, Martin Holborn.
Sociology – Anthony Giddens.
Social Problems in India – Ram Ahuja.
Society In India: Concepts, Theories and Recent Trends – Ram Ahuja.
Sociological Theory – George Ritzer.
Oxford Dictionary of Sociology.
Modernisation of Indian tradition – Yogendra Singh.
Sociological Thought – Francis Abraham, John Henry Morgan.
Social Change in Modern India – M.N. Srinivas.
Caste Its Twentieth Century Avatar – M.N. Srinivas
Persistence and Change in Tribal India – M.V.Rao.
Social Background of Indian Nationalism – A.R. Desai.
Anthropology.
Physical Anthropology – P. Nath.
Anthropology simplified - Vivek Bhasme.
Anthropology – Ember and Ember.
An introduction to social Anthropology – D. N. Majumdar and T.N. Madan.
An Introduction to Anthropological Thought – Makhan Jha.
Indian Anthropology – Nadeem Hasnain.
Indian Anthropology – R.N Sharma
Tribal India – Nadeem Hasnain.
State, Society and Tribes - Virginius Xaxa.
Xaxa Committee Report.
Philosophy.
History of Philosophy – Frank Thilly.
Survey of Indian Philosophy – C.D. Sharma.
Indian Philosophy, A counter Perspective – DR. Daya Krishna.
Indian Philosophy 2nd Volume - Dr S.Radhakrishnan.
Existentialism: A Reconstruction – David Cooper.
Existentialism Is A Humanism – Jean Paul.
An Introduction To Indian Philosophy – S Chatterjee.
A Critical Survey of Indian Philosophy – C Sharma.
A Critical History of Western Philosophy – Y. Masih.
Contemporary Western Philosophy – Dr B.K. Lal.
An Introduction to Political Theory – O.P Gauba.
Introduction to Religious Philosophy – Y. Masih.
Philosophy of Religion – John H. Hick.
Conclusion.
As one's selection is purely based on one’s optional subject, so choosing the right optional subject is mandatory. Moreover in General Studies papers, one cannot make much difference, here the optional subject is the key to making your slot in the interview list or final list. The key to studying an optional subject is to consolidate your basics and understand very well the whole optional subject. Prepare your notes meticulously and revise them as much as you can. Wish you the best of luck.