Hello

If you prepare for UPSC examination with honesty then one or two year is enough for preparation and now that you are in first year of graduation you can you start your UPSC preparation even now as syllabus is vast and if you start now you will be able to complete the syllabus

Top books which you can follow for UPSC examination preparation are listed below subject wise:-

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