Spy School Goes East: Book 14 by Stuart Gibbs

When the seasoned adult spies of the Hale family mysteriously disappear on a mission to China, Ben Ripley and his fellow spies-in-training head east to discover what happened to them—and find themselves up against a villainous plot that will take them from the skyscrapers of Shanghai to the ancient ruins of the Great Wall to the remote regions of the Gobi desert.