Hi Akshay
The DGCA Nav exam actually consists of the following subjects
1) General Navigation
2) Radio Navigation
3) Instrumentation
4) Performance
5) Mass and Balance
6) Flight Planning
Please read the DGCA syllabus thoroughly for the same
http://www.dgca.nic.in/cars/D7B-B4.pdf
The Indian DGCA follows the EASA ATPL syllabus for both CPL and ATPL . Keith Williams is nice but dont stick to it alone
I highly recommend the Oxford ATPL books as well
Good Luck