Currently I'm working on SharePoint 2013 as a developer. Please suggest me, where to get documents/reading material to crack 70-331 certification.
Just gonna give you some tips that helped me when studying for exams.
I would suggest watching this. It covers some, if you are insecure in how the exams are, and gives you an idea of what's coming on the exam.
The list is very "General". But from that you can fairly easily find various Technet and MSDN articles that covers the particular subjects.
Go to http://www.microsoftvirtualacademy.com/ for coverage on basic SharePoint learning videos.