Like all Biggles novels, it's very much of its time. Johns was probably fairly progressive for his time, with his racism more Kiplingian than Mosleyan. "These poor child-like unfortunates are British subjects [not citizens, obviously] and will be treated as such" etc. Live with that and we have a story clearly inspired by the discoveries of Inca and Maya cities, with a... let's say child-like glee at discovering "lost" civilizations that's pretty contagious. But seriously, can Ginger just fall off a cliff already?