18. "Ashenden: Or The British Agent" (1928) - W Somerset Maugham
Slightly different to the other entries on this list, W Somerset Maugham's "Ashenden: Or the British Agent" is a collection of loosely-linked short stories rather than a novel - and is largely based on his past experience as a member of British Intelligence on Continental Europe during the Great War. Having been dispatched to Switzerland under the false pretense that he was writing a play at the beginning of World War I, before moving to revolutionary Russia later on in the conflict, Maugham details how ruthless and brutal life as a British spy was - adding in some fictionalised treachery and debauchery to make it a real page-turner. So successful was Ashenden that Maugham became one of the best-paid writers of the 1930s, and Alfred Hitchcock then directed "Secret Agent" in 1936, a film loosely-based on two of the short stories, before the BBC did a four-part mini-series in 1991. Ever wanted to read about an Italian dancer inducing her Indian lover to cross the border from neutral Switzerland into Allied France? Well, here's your chance...