Hello

The best book for your CSAT or prelims examination for UPSC  is given below :-

HISTORY

NCERT XI (Ancient & Medieval)
NCERT XII (Modern Indian History)
Facets of Indian Culture Spectrum
From Plassey to Partition - A History of Modern India

Indian Art & Culture

Indian Art & Culture - Nitin Singhania

Polity

NCERT IX-XII
Indian Polity M Laxmikanth

Geography

NCERT VI X (Old Syllabus)
NCERT XI, XII (New Syllabus)
Certificate Physical Geography G C Leong
World Atlas

Economics

NCERT XI
The Indian Economy Sanjiv Verma
Indian Economy by Ramesh Singh
Economic Development & Policies in India Jain & Ohri

Environment

Class XII Biology (last 5 chapters)
Environment by Shankar IAS Academy
Environment & Ecology: Biodiversity, Climate Change and Disaster Management - General Studies for Civil Services (Main) Examination - Paper II and IV - Majid Hussain

International Relations

NCERT XII (Contemporary World Politics)
Current Affairs ( read newspaper as well)

To get the complete list of books go through
https://competition.careers360.com/articles/books-for-upsc-ias

Thank you