The Ultra Secret is a personal account of some of the cryptographic exploits of the Second World War. Winterbotham was an intelligence officer in charge of Ultra, the Bletchly Park effort to break the Nazi codes. A former RAF pilot, he is proudly not a mathematician, and his explanation of code breaking is a mysterious 'bronze goddess' who spits out decrypts. Rather, Winterbotham handled translating and distributing messages to allied commanders, a system named Ultra run by a world-spanning network of Special Liaison Units. He saw himself as a 'shadow OKW', a British version of the Nazi command staff, which enabled Allied generals to fight with the enemy's cards on the tables.
This was an incalculable advantage. With Ultra, Churchill and Dowding knew the margins of Luftwaffe endurance in the Battle of Britain, and could hold out until the window for Operation Sea Lion had passed. Ultra outfoxed Rommel in North Africa, and reassured Allied commanders that deception plans for amphibious landings, including Overlord, were working.
The book is at its best when it goes to Winterbotham's personal judgement of character. He knew most of the senior Nazis from his time as an attache in Germany in the 1930s, where he gathered vital intelligence by asking questions and letting Hitler talk (loose lips sink conquests). He praises many generals, Patton especially, for bold use of Ultra to punch around concentrations. Montgomery earns mixed marks as a traditionalist who fought in deliberate ignorance of Ultra, and Mark Clark, commander in Italy, dramatically failed to exploit Ultra several times at the expense of his troops.
This book was probably stunning when it was published. Now it's light and short on details, a conventional WW2 history. Compared to R.V Jones' Most Secret War, which drips with personal insights and interest, The Ultra Secret is an anodyne, bureaucratic history.