That's the name of a book written by a chap called Daniel Everett, who's been living among the Piraha tribe in the Amazon basin of Brazil. Their spirits are interesting - according to Wikipedia, they include visible, tangible things, like trees or jaguars or whatever. One of the most bizarre aspects of the Piraha is their language, which is causing massive controversy among linguists. Apparently it doesn't have: Words for please, thank you or sorry. Numbers. Most controversially of all, they don't appear to have recursion. So, they can handle "That is red." They can handle "That is a butterfly." They will not say "That is a red butterfly." This has gotten the linguistic world all fired up, since Noam Chompsky argues that humans have a universal grammar, which definitely includes recursion. The whole thing's damn fascinating. The simple fact that they converted the missionary is simultaneously awesome and hilarious, given the terrible history of missionary interaction with people around the world.