Hi,
Following are the best books for preparing for UPSC Prelims:
History
Indias Struggle for Independence Bipan Chandra
Facets of Indian Culture Spectrum
NCERT XI (Ancient & Medieval)
NCERT XII (Modern Indian History)

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

Indian Polity
Indian Polity M Laxmikanth
NCERT IX-XII

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

International Relations
NCERT XII (Contemporary World Politics)
Current Affairs

CSAT
Tata McGraw Hill CSAT Manual
CSAT II Arihant
Verbal & Non-Verbal Reasoning R S Aggarwal

NCERT books from 6th to 12th are a must.