This book comes with my son's highest rating: "It's pretty good."
A young boy pining to be an explorer, a pessimistic girl, and a 5-year-old boy and his older sister crash land a plane in the Amazon. They encounter bugs and caymans and fire and monkeys and all manner of obstacles on their hike out -- until they meet an unlikely rescuer, the "Explorer."
Read with a keen eye and memory (Hint: The information about the historical explorers is vital!) to catch the "twist" at the end -- when the Explorer's identity is finally revealed.