Arthur Ransome 
Pigeon Post 

Reunited for the summer, the Swallows and Amazons with Dick and Dorothea launch a prospecting expedition to find the lost gold mine of the high hills above the lake. But the mining camp runs into all sorts of trouble: not only the danger of fire in the drought ridden countryside but also scary encounters with unsafe tunnels. Worst of all is the sinister Squashy Hat, who appears to be a rival prospector and who”s certainly a spy – how can they keep working without him discovering what they”ve found?