Daniel Everett went to Brazil as a missionary to convert the Pirahã, a tiny (<400 members) Amazonian tribe. Instead, the tribe effectively converted him into an atheist. He then became a professional linguist and anthropologist, and has continued to study the tribe.
Don’t Sleep, There are Snakes is fascinating for its description of the Pirahã culture/language, which is so dramatically different from ours as to radically challenge our notions of language and even what it means to be human. They lack numbers, words for colors, Gods, and creation myths. They don’t have words for “right” or “left” – instead, they might refer to your “upriver” leg. Their tonal language has three vowels and eight consonants (seven for women), although they can whistle and hum the language, useful for hunting and talking while your mouth is full, respectively.
Their language has no recursion, which makes Noam Chomsky cry. One can only legitimately talk about things one has directly experienced, or things that someone who directly experienced them told one. This made Everett’s proselytizing very difficult, since he had never met anyone who had seen Jesus. Because they don’t have numbers, they can’t do arithmetic. At all. They have the simplest kinship system known, no war, and like many hunter-gatherers, no system of private property.
The book also includes some very lively anecdotes about river traders, malaria, etc., but for me the overwhelming value was as an ethnography that made me marvel at the diversity of human culture.