This scholarly work by the noted linguist Suniti Kumar Chatterjee was first published in 1951. There is a newer edition from 1974. The Kiratas - or the Indo-Mongoloids as Chatterjee calls them - are a group of people who originally inhabited the mountaineous regions (particularly the Himalayas and the North-East of India). They are mentioned in the ancient Indian epics, and by Greek and Roman historians. The Mahabharata talks about King Bhagadatta of Pragjyotishpur (ancient Assam) who fought on the side of the Kauravas with his army of "Kiratas", "Chinas", and those who "dwell by the sea".
Chatterjee's treatment of who the Kiratas were, and their contribution to the history and culture of the north-eastern and eastern regions of India, and India as a whole, is far broader than what Wikipedia defines them as. He devotes well-defined sections to their possible origins, the possible migration routes through which they entered India at various points in history, and settled and intermingled with existing people, giving rise to different cultures. The gradual "Sanskritization" - i.e. inclusion and synthesis of the tribal cultures into Hinduism - a process which went in both directions - involved the identification of tribal customs, rituals, and deities with Hindu counterparts. This is similar to the process in which many pagan traditions were merged into Christianity to make it acceptable in its early days.
Although Chatterjee's style of writing is evidently a bit archaic, this book is a wealth of well-researched, albeit out of date in some places, information on the topic.
This book is freely available on archive.org and I found it hard to find a physical copy.