We Are Not Eaten by Yaks - C. Alexander London

Eleven-year-old twins Oliver and Celia Navel could care less about adventure and they really do not like excitement. They'd rather be watching television. Unfortunately for them, their thrill-seeking parents have dragged them from continent to continent their entire lives. But when their mother goes missing and their father makes a bet with the devious explorer Sir Edmund, the twins are forced into action. They head to Tibet where they fall out of airplanes, battle Yetis, poison witches, and encounter one very large yak.
If they can unravel the mysteries and outwit Sir Edmund, they might just make the discovery of a lifetime . . . and get cable television!