Nowadays, there are lots of resources available online to prepare for IAS.
Following are the books recommended to prepare for IAS -History
History Of Modern India - Bipan Chandra.
India's Struggle For Independence - Bipan Chandra.Culture
The Wonder That Was India - A.L. Bhasham.
Indian Art and Culture - Nitin Singhania.Geography
Geography of India - Majid Husain.
Oxford School Atlas - Oxford.Polity
Indian Polity for Civil Services Examinations - M. Laxmikanth.Economy
Indian Economy - Ramesh Singh.
Economic Survey.Science & Technology
Science and Technology in India - TMH. Environment
Environmental Studies: From Crisis to Cure - Rajagopalan.
Environment for Civil Services Prelims and Mains - Khuller. Current Affairs
India Year Book
Manorama Yearbook.CSAT Paper 2
CSAT Paper - 2 Manual by TMH or CSAT-II - Arihant Analytical Reasoning
Analytical Reasoning - M. K. Pandey Reasoning
Verbal & Non-Verbal Reasoning - R. S.Aggarwal
For UPSC preparation, there are various websites providing free study material for Civil services
Following are some tips for IAS Aspirants -1. Take the early advantage
2. Realise that you have time constraints, so prioritize your time!
3. Buy the most recommended books for IAS preparation
4. Enroll for mock test series
5. Read newspapers and magazines
6. Discuss current issues with your friends
Hope the answer helps.