Which extra subjects do I have to study if I prepare for the HCS along with the UPSC CSE?

The HCS examination tests the candidate knowledge on the state of Haryana in addition to the nation as a whole. Thus if one is sincerely preparing for the Civil Services Examination, the topics on Geography, History and Current Affairs related to the state of Haryana should be overviewed alongside.
The Haryana Civil Services Exam involves a three-stage selection process consisting of :

a) Preliminary Examination: The examination consists of 2 papers i.e General Studies and Aptitude Test. The Preliminary examination comprises of two papers General Studies Aptitude Test of 100 marks each.
b) Mains – 5 papers (English, Hindi, General Studies, Subject 1 and Subject 2)
c) Personality Test

The main difference in a State HCS is the increased focus on the state. The rest of the process is almost similar to the UPSC. Books like Haryana Samanya Gyan or any other Haryana General Knowledge book is all that you need in order to cover the entire syllabus of Haryana General Knowledge.

The book covers extensively on Haryana Administration, Polity and current events relating to the government.Any book for Aptitude for the Civil Services Examination covers all the important topics including regarding data interpretation, Quantitative Aptitude, and Mental Ability.

Some telegram groups can be joined for free subject material and have a discussion with other aspirants.