In the 1970’s the first books came out that revealed how in WWII the Allied codebreakers were able to solve the German Enigma cipher machine (and other systems). Those early books were written without access to all the official documents and many of the things they claimed were incorrect. They also greatly exaggerated the effects of codebreaking on the various campaigns of the war.
Unfortunately historians have mostly relied on those early books so most history books continue to claim that the Allies won specific battles or campaignsbecause they could read German messages etc.
If you are looking for a more reliable analysis of the role that secret intelligence played in WWII then you need to read the official ‘British Intelligence in the Second World War’ volumes:
Volume I: Its Influence on Strategy and Operations, 1979
Volume II: Its Influence on Strategy and Operations, 1981
Volume III, Part 1: Its Influence on Strategy and Operations, 1984
Volume III, Part 2: Its Influence on Strategy and Operations, 1988
Volume IV: Security and Counter-Intelligence, 1990
Volume V: Strategic Deception, 1990
These books are large and heavy with many chapters and appendices, covering all the campaigns of the war and the role that intelligence played.
They’re also out of print so you’ll have to go to a used books store and be prepared to pay a premium. If you do get hold of them however you will undoubtedly be impressed by their scope and analysis.
I have the first four books on operations and each volume covers a specific time period. There are separate chapters for the organization of the intelligence agencies, economic and strategic assessments and of course the role that intelligence played in the actual campaigns.
Overview
1). The first volume covers prewar intelligence arrangements and ends with operation Barbarossa in the summer of 1941. The campaigns of Norway, France, Battle of Britain, Battle of the Atlantic, the Balkans and N.Africa are covered. Among the most interesting appendixes are: appendix 1 which covers the Polish and French contribution to the solution of the Enigma machine (with mistakes that were corrected in vol3), appendix 9 ‘Intelligence in advance of the GAF raid on Coventry’ and appendix 11 ‘GAF Navigational Aids’.
2). The second volume covers the British strategic assessments from mid ’41 to mid ’43, the War at sea up to summer ‘43 and most importantly the North African campaign from July 1941 till the fall of Tunisia. Some of the appendices have interesting information on the security of British ciphers (appendix 1), the German police ciphers (appendix 5), the British assessment of German tanks and A/T guns (appendix 14) and the compromise of the initial German plan for the battle of Kursk (appendix 22).
3). Volume 3 part 1 has the strategic assessments from June ’43 to June ’44, the British assessments of the German war economy, the Italian campaign, the developments in occupied Yugoslavia (Chetnik-Partisan conflict), the War at sea from summer ’43 to summer ’44, the air war in the West and the intelligence on the V-weapons (V1 flying bomb and V2 rocket). There is interesting information in appendix 2 describing the solution of the Tunny teleprinter, in appendix 4 ‘Intelligence estimates and German statistics on the German war economy’, in appendix 10 ‘’German anxieties about Allied ability to locate U-boats’ and appendix 16 ‘Decrypt of the Japanese ambassador’s report to the foreign ministry Tokyo of his interview with Field Marshal Milch in Berlin, 17 August 1943’.
4). Volume 3 part 2 is the largest book (1.038 pages!) and covers the planning and execution of operation ‘Overlord’, the fighting in the Eastern Front, the Italian campaign, the War at sea and the Allied strategic bombing offensive up to the defeat of Germany. There is interesting information in appendix 9 ‘Intelligence on Germany’s reinforcement of the Cotentin peninsula and its effects on First US Army’s operational plans’, appendix 10 ‘Allied intelligence on German divisions on the Eve of D-day’, appendix 14 ‘Intelligence relating to 21st Panzer division and 352nd Infantry division up to D-day’, appendix 26 ‘Intelligence on the Axis oil situation up to the summer of 1944’, appendix 29 ‘TA project: Enemy intelligence’ (nuclear weapons research) and appendix 30 ‘Polish, French and British contribution to the breaking of the Enigma: A revised account’.
As in all books there are some small mistakes (or white lies) especially regarding the influence of codebreaking on actual operations (for example on the sinking of Rommel’s supplies). However these are nothing compared to the monstrosities that one reads in other history books.
Overall the ‘British Intelligence in the Second World War’ volumes remain the most authoritative source on signals intelligence in WWII.