Mary Pope Osborne 
Magic Tree House 6: Adventure on the Amazon 

Eight-year-old Jack and his little sister, Annie, are playing in the woods when they find a mysterious tree house full of books. But these are no ordinary books . . . And this is no ordinary tree house . . .
Jack and Annie might be on their scariest adventure yet – on the Amazon river! There are army ants, jaguars and piranas, as well as vampire bats, snakes and even crocodiles! And they”ve still got to find another clue to help rescue Morgan, the owner of the Magic Tree House. For a while, Jack and Annie are worried that they might be stuck in the rain forest for ever . . . But the over-active monkey they meet turns out to be friendlier than they first thought!