Being a non-Law graduate, we recommend you to consider Law as optional subject if you are able to understand the subject and related requirements or can get proper guidance.
However, some of the books suggested are available in Hindi also but, Law in Hindi medium is not that popular optional subject in CSE.
To begin with, we suggest following books for preparing Law as optional subject in Civil Services (Main) Examination.
The Constitution of India P. M. Bakshi
Law Of Torts With Consumer Protection Act R K Bangia
Contract Avtar Singh
Indian Penal Code
Bare Act Bakshi
STARKE's INTERNATIONAL LAW