The FAA has a few books of its own that i found to be extremely helpful during my training. start of with the pilot's handbook of aeronautical knowledge
Pilot's Handbook of Aeronautical Knowledge.
This will give you a very good idea of where to begin. studying for the DGCA exams should not be difficult if you follow what everyone has been talking around this forum. look up the DGCA website for suggested books too, dont know if they have updated it but the GSP's out there are excellent too.