Operation Mincemeat by Ben Macintyre

The subtitle is 'How a Dead Man and a Bizarre Plan Fooled the Nazis and Assured Allied Victory'. Which pretty much sums up the book's contents.
The tale of the transformation of an "unknown" corpse into the fictitious Captain William Martin – whose body, complete with an entirely invented past life (theatre stubs, love letters) and, crucially, misleading information on the forthcoming invasion of Sicily, was deployed, apparently drowned, into the sea off Spain in 1943 as a "Trojan horse" to find its way back to German intelligence – was the basis of the 1956 film The Man Who Never Was. But as with so many films and books in the two decades following the war, propagandising and officialdom prevented the entire story from coming to light: Macintyre, by means of extensive sleuthing – there are more than 30 pages of impeccable annotated notes – and a fortuitous visit to the son of intelligence officer Ewen Montagu, one of the main players possessed of the necessary "corkscrew mind", gives the final word on this extraordinary episode.
It was, ultimately, a success. Hitler was persuaded that the Mediterranean offensive would come at Greece and Sardinia, and any attack on Sicily would be a feint. His defences were radically, disastrously shifted and the rest is history. It was a way to avoid the still well-oiled German war machine. They needed a bit of good fortune, which they got.
But a huge part of the story is the strange men in British intelligence, and the strange world they inhabited, who were behind the planning, including Ian Fleming. They appear, in this book, to have lost their perspective on what would constitute normalcy--they were entirely wrapped up in the spy and subterfuge efforts. True, they needed some luck, but they also displayed uncanny adroitness, not just in the selling of deception but in its after-sales care.
Great story well told.