Help me find a book through which I can understand role of NE in INDIA'S struggle for freedom. I also want to know motives of NE leaders to join the resistance, what were they expecting?

Note: tried posting it on r/india, indiaspeaks but they removed it 😢 I want to know how INDIA became INDIA as we know now. There were different empires but no one ever fully united India as India is now. Like Marathas did not captured many states of India and Nor did Mughals captured whole of India etc. Northeast was ruled by Kamrupa empire which was subordinate to Gupta Empire but after that Ahoms came here and settled, although a foreign power but they later became a part of Indian Empires because they mixed in with people. To understand this very complex thing, the Idea I need a book which can tell me how states were brought together and on what basis. If I could understand the reasons why different leaders joined the freedom struggle would that be helpful? Or should I go for some book by SARDAR VALLABHBHAI PATEL, as he was the one to unite all states into India. Please suggest me some proper books, resources or tell me if you know how it happened.