Well, keep in mind first that the full title of the story is Bruchko: The Astonishing True Story of a 19-Year-Old American, His Capture by the Motilone Indians and his Adventures in Christianizing the Stone Age Tribe. This title, of course tells us a lot about the story.
In short, this is a book about Bruchko, a Christian called to work with the natives of South America. He himself has a strange conversion story from his youth. After his own church won't recognizes his encounters, Bruchko begins to feel called to be a missionary to the Motilones (that is the tribe of natives spoken of above) who are located near both Venezuela and Colombia.
The next part of the story is about Bruchko's struggles to be accepted among the tribe. Bruchko endures everything from mishaps to misfortune, from disease to torture. However, Bruchko is finally accepted by the tribe. Through his work with them Bruchko learns three important things: Christ is perfectly able to head His own Church, Christ doesn't need culture to be negated, and that (unfortunately) suffering is always a part of God's plan for humans.
In conclusion, one can expect an interesting reading of a beautiful missionary story.