During World War II, Britain enjoyed spectacular success in the secret war between hostile intelligence services, enabling a substantial and successful expansion of British counter-espionage. But these victories were kept secret for many years, emerging only gradually and in a piecemeal way.

Hugh Trevor-Roper's experiences working in the Secret Intelligence Service (MI6) during the war had a profound impression on him and he later observed the world of intelligence with particular discernment. To Trevor-Roper, who was always interested in the historical dimension of the present and was fully alive to the historical significance of the war through which he lived, the subject of wartime intelligence was as worthy of profound investigation and reflection as events from the more-distant past. Expressing his observations with his former colleagues through some of his most ironic and entertaining correspondence, Trevor-Roper wrote with a freedom he could not express publicly due to the Official Secrets Act. The coherence, depth and the historical vision which unites these letters can only be glimpsed when they are brought together from the scattered publications in which they appeared, and when read beside his unpublished, private reflections.

The Secret World unites Trevor-Roper's writings on the subject of intelligence – including a unique collection of Trevor-Roper's personal letters to a wide cast of leading figures in government, the military and the secret service and an extraordinary chain of correspondence with the exiled spy Kim Philby. Based on original material and extensive supplementary research by E.D.R Harrison, this book is a sharp, revealing and personal first-hand account of the intelligence world in World War II and its aftermath.