DBWI: Unify India

I think they a one book, in which India was unify, but it was divide between Hindu, and Muslim.

Oh, you mean the one where the Muslim portions of India broke away, developed nuclear weapons and then started actively aiding terrorists like some sort of Libya on steroids? That was a silly book, no way that scenario actually plays out in real life.
1. Have the war going worse for Britain so it needs more Indian troops to combat Japan and/or Middle East and so is more lenient in dealing with dissent.
2. Britain not only promises but actually delivers Dominion Status when the war ends.
3. Appoint a last Viceroy who actually knows what he is doing instead of being a political apparatchik (Mountbatten was mentioned as a possibility but didn't get it as he was considered too closely related to the Royal family but also he was perhaps too well known for calling spades spades and not suffering fools especially political ones)
4. Make it known that any separatist movements will be strongly discouraged.(The viceroy actively encouraged Hyderabad and Burma to go their own ways).
Then you will probably get a larger unified country from India (however I'm not sure about it being all of the Raj)