Hello Aspirant,

For UPSC prilims subject wise preparation the best sourse is NCERT boooks of 11th and 12th standard. For the civil service aptitude preparation the following are the best recommended :

Tata McGraw Hill CSAT Manual

CSAT II – Arihant

Verbal & Non-Verbal Reasoning – R S Aggarwal

Cracking the CSAT Civil Services Aptitude Test with Solved Papers

The Pearson CSAT Manual

For mock test series there are many good websites iasgateway and apps like Gradeup and Unacademy with paid material . If you search further there are few free ones too but msotly for good quality test series you have to pay.If you search well , you will come across many such apps.

The following links may help you :

https://competition.careers360.com/articles/books-for-upsc-ias

https://competition.careers360.com/articles/how-prepare-for-upsc-ias-exam-online

https://competition.careers360.com/articles/how-prepare-for-upsc-ias

Good luck