We Are Not Eaten by Yaks — C. Alexander London

Eleven-year-old twins Oliver and Celia Navel live on the 4-1/2th floor of the Explorers Club with their father, Dr. Navel. Their mother, Dr. Navel, has been missing for years. So when an explorer shows up with a clue as to where his wife could be, Dr. Navel drags Oliver and Celia to Tibet to find her. Once there, the twins fall out of airplanes, encounter Yetis, travel through waterfalls, and end up in the Demon Fortress of the Warrior King where they -- just possibly -- might find their mother and save their father from the Poison Witches. Thing is, they would much rather be watching television. And if their trip doesn't work out as planned, the twins could end up as slaves to Sir Edmund Thitheltorpe III, an evil explorer with breath that smells like boiled carrots, who has it in for the whole Navel family.