About the book (from Wikipedia):87
The Valley of Adventure (published in 1947) is a popular children’s book by Enid Blyton. It is the third book in the Adventure Series. The first edition of the book was illustrated by Stuart Tresilian.
Preparing for a trip by plane with Bill, the four children never mind to inadvertently go into the wrong plane, which takes off under frightening circumstances — and find themselves stranded in an isolated valley scarred by war, but they did not care and at the mercy of sinister men looking for a long-lost treasure.They find a man called Otto, who was captured by the men. He draws a map for Jack showing the nearest district.