I was sorting out some photos of my trip to the U.K., and this one made me laugh. It was in the window of The Haunted Bookshop in St Edward's Passage in Cambridge.
The book is summarized here. W. E. Johns is given the rank F/O (Flying Officer) on the cover. The book dates from 1932, the year the first 'Biggles' book appeared (we've just missed the 80th anniversary of The Camels Are Coming). Johns published as "Captain", but never in fact held that rank.