Late Lunch with Llamas by Mary Pope Osborne
When the magic tree house whisks Jack and Annie to a mountainside, they are surprised to find farmers nearby. Then they learn that the farmer’s baby llama has been stolen, and Jack and Annie vow to bring the little animal back to its owners. But the journey is treacherous–they must climb to the peak of Machu Picchu and climb back down in order to complete this mission. Jack and Annie have been on many dangerous travels, but can they survive this one?