Nicola Barber 
Who Broke the Wartime Codes? [PDF ebook] 

How do we know about the men and women who broke important World War II military and diplomatic codes? What were the challenges, and what happened to them? This book shows how we know about the Allied codebreakers and their experiences from primary and other sources. It includes information on some historical detective work that has taken place, using documentary, archaeological, and oral evidence, that has enabled historians to piece together the fascinating story of those who provided top secret information known as Ultra and Magic.