Operation Storm City (The Guild of Specialists #3) by Joshua Mowll

INDIA, 1920: Becca and Doug continue to search for their missing parents; a quest which takes them deep into the Sinkiang - the Desert of Death.

SINKIANG DESERT, CHINA: Forces converge in the race to find Ur-Can - the fabled Storm City. Can the Guild get there before General Pugachev takes control and endangers the whole planet?

UR-CAN - STORM CITY: Deep in the desert wastes, the ancient underground city is stirring back to life. Have Doug and Becca learnt enough to solve the final mystery, or will they find their parents only to lose them for ever?