All the works included in this collection are dedicated to the work of intelligence services during the First World War and were written by employees of these services. For Basil Thomson, head of the Scotland Yard's Criminal Investigation Department during the First World War, there were almost no secrets that he was not privy to. Throughout the four years of the war, he was in constant contact with statesmen from many countries. His book is dedicated to the secret side of the war. R. Boucard's book provides a fascinating insight into the techniques and methods of the Allied intelligence services during the First World War. Louis Rivière dedicated his work to the activities of the German secret service in Madrid.