We can find a lot in text books and Wikipedia about Assam History, but then text books are written by Government and biased while Wikipedia is edited by public.
Came across the Encyclopedia Britannica article on Assam and straight forward, crisp writing on Assam History.
https://www.britannica.com/place/Assam/History
Excerpt from Britannica, Read the complete article, it is crisp, short and has all the incidents covered.
In the earliest recorded times, Assam was part of Kamarupa, a state that had its capital at Pragjyotishapura (now Guwahati). Ancient Kamarupa included roughly the Brahmaputra River valley, Bhutan, the Rangpur region (now in Bangladesh), and Koch Bihar, in West Bengal state. King Narakasura and his son Bhagadatta were famous rulers of Kamarupa in the Mahabharata period (roughly 400 BCE to 200 CE). A Chinese traveler, Xuanzang, left a vivid account of the country and its people about 640 CE. Although information about the following centuries is meagre, clay seals and inscriptions on copper plates and stone that date from the 7th to the mid-12th century indicate that the inhabitants of the region attained considerable power and a fair degree of social, economic, and technological development.
