Mystery of the desert giant

Juvenile fiction.
After the disappearance of two men who were flying over the California desert, the Hardy boys investigate the ancient Indian drawings of giants that may have played a role.
The Hardy Boys and Chet Morton search on the California desert for missing industrialist, Willard Grafton, and break up a gang of criminals intent on defrauding the US government.
This book opens with Frank, Joe & Chet lurking in the front garden of the Hardy home with Chet's infrared camera. They've received a phonecall about a mysterious visitor & are determined to get a picture of him. One man turns up, but then walks away without attempting to knock on the door. A second "visitor" then arrives. He proves to be Philip Dodge, a lawyer that their father knows. He tells the Hardys that his client, a Mr. Brownlee, is searching for his nephew, Williard Grafton, who, along with a friend called Clifford Wetherby, has vanished. Fenton Hardy tells Dodge that he's occupied with another case but that Frank & Joe are more than capable of handling the case. Meanwhile, Chet has been developing the infrared photos in the Hardys' lab, but on the way back to the house he's knocked out. It soon becomes apparent that a mysterious man has eavesdropped on the conversation between the Hardys & Dodge & has presumably heard all of their discussions relating to Grafton & Wetherby. The three boys are soon on their way to California to investigate the disappearance of Grafton & Wetherby, and it soons become apparent that there's a gang determined to thwart their endeavors. Their investigation leads them to Mexico & Colorado as well as California but the final showdown comes at the titular desert giant when Frank & Joe fall into the hands of the gang. - dooyoo.co.uk reviewer keeperofthematri.