I read this while I was visiting The Andaman Islands, part of the archipelago to which North Sentinel Island is also connected. It lies between India and Thailand and is ruled by India even though, geographically it is closer to Thailand.
The Sentinelese are totally isolated from the modern way of life, still hunter-gatherers, they have yet to develop any form of cultivation or farming. Although pigmy sized and African in appearance, genetic studies have shown the presence of other minor genes.Contact was made with them a few decades ago but is now totally prohibited as our diseases could wipe out their fragile community.
This book is a truly fascinating novel, based around the expertise of Mr Daglish, an anthropologist. Unfortunately the English is chronic, often incomprehensible, and you would have to be seriously interested in the subject to keep with it. For me, the rewards were worth the effort. There are several museums on the Andaman Islands relating to the Sentinelese and other nearby tribes, and having read this book it boosted my understanding of the displays.
The title, Guardians of the Fire, refers to the fact that the Sentinelese do not make fire, but they guard the fire that they have obtained from lightening strikes in the past, splitting it and keeping reserve in case of need. They collect iron that floats in on wooden objects and use this to make tips for their spears and they consider coconuts to be a delicacy, a gift from the seas. Absolutely fascinating.