Some of the common sources useful for UPSC Civil Services Exam are listed below.

General Studies - Prelims & mains

Ancient India – Old NCERT

ICSE – Environment – Class 9

World History – Arjun Dev 

ICSE – Environment – Class 10

Modern India – Old NCERT

Public Policy – Basics – Must Read for GS – 2, 3 &  4

Medieval India – Old NCERT

Sociology

Indian Society – Sociology – Paper 2

Sociology – Fundamentals

Political Science and INTERNATIONAL RELATIONS

Indian Polity – Must Read

International Relations – Basics

Political Theory – Basics