This is a facinating book, by Lt. Cmdr. Ewen Montagu who worked for British Inteligence during the second world war. He was involved in many covert activities notably;
The Double Cross System, where the British ran all the spies that the Germans sent to the Uk.
Operation Mincemeat, where a human cadaver carried false documents (and letters) was put into the sea near Spain. These documents succesfully convienced the Germans that an Alied attack would be made against Sardinia and Greece, instead of the real target Sicily.
A film, ’The Man Who Never Was’, dramatised this operation. One notable change was made to the plot, was the way the Alies confirmed that Germany had taken the bate. In the film, an Irish spy was send to the UK, whereas in reality due to the Double Cross System, the Alies already knew.
Note: Cmdr. Montagu makes a cameo in the film …