Coles, Manning - A Drink to Yesterday (1940)

Probably because it was based on the real experiences of one of it's co-authors this book was immediately hailed on publication as something new in spy fiction. Gone was the old style fanciful idea of the secret agent as a romantic individual, to be replaced by the professional intelligence agent, working in collaboration with his nation's armed forces as part of a team effort. Set in WWI it gives a good idea of what it must have been like to be a young man recruited into actual espionage work in that war. Bill Saunders is that young man, recruited by one of his teachers, Tommy Hambledon and we follow in considerable detail his search for a German spy. Somewhat more grim than later books in the series it still has flashes of humour and is notable for it's description of conditions in Germany at that time.