Ashok Kumar books are quite well known. Dr Lallan ji singh book is also recommended. you can also read Bharat ki videsh niti by Dr R S Yadav. For English you can refer to Pearson as it will help you enhance your vocabulary and grammar too.

Some other well known books that are recommended for this exam are Theory and Practice of war, probing war by J A Khan and Military history and Modern Strategic thought from Machiavelli to Hitler, National Security.

Best wishes. Thank you.