Ultra adalah sebuah rancangan yang diadopsi oleh intelijensi militer Britania Raya pada Juni 1941 untuk intelijensi sinyal masa perang yang dilakukan dengan membongkar komunikasi radio dan teleprinter musuh terenkripsi secara tingkat tinggi di Government Code and Cypher School (GC&CS), Bletchley Park. Ultra kemudian menjadi rancangan standar di kalangan Sekutu barat untuk seluruh intelijensi semacam itu.
- Bamford, James (2001), Body of Secrets, Doubleday, ISBN 0-385-49907-8
- Bennett, Ralph (1999) [1994], Behind the Battle: Intelligence in the War with Germany (edisi ke-Pimlico: New and Enlarged), London: Random House, ISBN 0-7126-6521-8
- Bertrand, Gustave (1973), Enigma ou la plus grande énigme de la guerre 1939–1945 (Enigma: The Greatest Enigma of the War of 1939–1945), Paris: Librairie Plon
- Beesly, Patrick (1977), Very Special Intelligence: The Story of the Admiralty's Operational Intelligence Centre 1939–1945, Sphere Books Limited, ISBN 0-7221-1539-3
- Budiansky, Stephen (2000), Battle of wits: The Complete Story of Codebreaking in World War II, Free Press, ISBN 978-0-684-85932-3 A short account of World War II cryptology which covers more than just the Enigma story.
- Brzezinski 2005 (missing citation)
- Calvocoressi, Peter (2001) [1980], Top Secret Ultra, Kidderminster, England: M & MBaldwin, ISBN 978-0-947712-41-9
- Churchill, Winston (2005) [1949], The Second World War, Volume 2: Their Finest Hour (edisi ke-Penguin Classics), hlm. 529, ISBN 978-0-14-144173-3
- Copeland, Jack (2004), "Enigma", dalam Copeland, B. Jack, The Essential Turing: Seminal Writings in Computing, Logic, Philosophy, Artificial Intelligence, and Artificial Life plus The Secrets of Enigma, Oxford: Oxford University Press, ISBN 0-19-825080-0
- Farago, Ladislas (1974) [1971], The game of the foxes: British and German intelligence operations and personalities which changed the course of the Second World War, Pan Books, ISBN 978-0-330-23446-7 Has been criticised for inaccuracy and exaggeration
- Farley, R. D. (25 November 1980), Oral History Interview NSA-OH-40-80 with Arthur J. Levenson (PDF), diakses tanggal 24 September 2016
- Ferris, John Robert (2005), Intelligence and strategy: selected essays (edisi ke-illustrated), Routledge, ISBN 978-0-415-36194-1
- Gannon, Paul (2006), Colossus: Bletchley Park's Greatest Secret, London: Atlantic Books, ISBN 978-1-84354-331-2
- Hanyok, Robert J. (2004), Eavesdropping on Hell: Historical Guide to Western Communications Intelligence and the Holocaust, 1939–1945 (PDF), Center for Cryptographic History, National Security Agency
- Hinsley, F. H.; Stripp, Alan, ed. (1993), Codebreakers: The inside story of Bletchley Park (edisi ke-OU Press paperback), Oxford: Oxford University Press, ISBN 978-0-19-280132-6
- Hinsley, Francis Harry (1993), British intelligence in the Second World War, Cambridge: Cambridge University Press, ISBN 978-0-521-44304-3
- Hinsley, Sir Harry (1996) [1993], The Influence of ULTRA in the Second World War (PDF), diakses tanggal 23 July 2012 Transcript of a lecture given on Tuesday 19 October 1993 at Cambridge University
- Hunt, David (28 August 1976), "The raid on Coventry", The Times, hlm. 11
- Jones, R. V. (1978), Most Secret War, London: Book Club Associates, ISBN 978-0-241-89746-1
- Kahn, David (1997) [1967], The Codebreakers: The Comprehensive History of Secret Communication from Ancient Times to the Internet (edisi ke-2nd Revised), New York: Simon & Schuster, ISBN 978-0-684-83130-5
- Kahn, David (29 December 1974), "Enigma Unwrapped: Review of F. W. Winterbotham's The Ultra Secret", New York Times Book Review, hlm. 5
- Knightley, Phillip (1986), The Second Oldest Profession, W.W. Norton & Co, ISBN 0-393-02386-9
- Kozaczuk, Władysław (1984), Enigma: How the German Machine Cipher was Broken, and how it was Read by the Allies in World War Two, edited and translated by Christopher Kasparek [a revised and augmented translation of W kręgu enigmy, Warsaw, Książka i Wiedza, 1979, supplemented with appendices by Marian Rejewski, Frederick, MD, University Publications of America, ISBN 978-0-89093-547-7 This is the standard reference on the crucial foundations laid by the Poles for World War II Enigma decryption.
- Lewin, Ronald (2001) [1978], Ultra goes to War (edisi ke-Penguin Classic Military History), London: Penguin Group, ISBN 978-0-14-139042-0 Focuses on the battle-field exploitation of Ultra material.
- Mallmann-Showell, J.P. (2003), German Naval Code Breakers, Hersham, Surrey: Ian Allan Publishing, ISBN 0-7110-2888-5, OCLC 181448256
- Momsen, Bill (2007) [1977], Codebreaking and Secret Weapons in World War II: Chapter IV 1941–42, Nautical Brass, diarsipkan dari versi asli tanggal 2001-01-26, diakses tanggal 2008-02-18
- Pidgeon, Geoffrey (2003), The Secret Wireless War: The Story of MI6 Communications 1939–1945, St Leonards-on-Sea, East Sussex: UPSO Ltd, ISBN 1-84375-252-2, OCLC 56715513
- Rejewski, Marian, wrote a number of papers on his 1932 break into Enigma and his subsequent work on the cipher, well into World War II, with his fellow mathematician-cryptologists, Jerzy Różycki and Henryk Zygalski. Most of Rejewski's papers appear in Kozaczuk 1984
- Rejewski, Marian (1984), "Summary of Our Methods for Reconstructing ENIGMA and Reconstructing Daily Keys, and of German Efforts to Frustrate Those Methods: Appendix C", dalam Kozaczuk, Władysław; Kasparek, Christopher; Frederick, MD, Enigma: How the German Machine Cipher Was Broken, and How It Was Read by the Allies in World War Two (edisi ke-2), University Publications of America, hlm. 241–45, ISBN 978-0-89093-547-7
- Andrew Roberts (2009). The Storm of War: A New History of the Second World War. Penguin Books Limited. hlm. 501. ISBN 978-0-14-193886-8.
- Schlesinger, Arthur, Jr. (1992), "The London Operation: Recollections of a Historian", dalam Chalou, George C., The Secrets War: The Office of Strategic Services in World War II, Washington, DC: Government Printing Office, ISBN 978-0-911333-91-6
- Singh, Simon (1999), The Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptography, London: Fourth Estate, ISBN 1-85702-879-1 This provides a description of the Enigma, other ciphers, and codes.
- Smith, Michael (2007) [1998], Station X: The Codebreakers of Bletchley Park, Pan Grand Strategy Series (edisi ke-Pan Books), London: Pan McMillan Ltd, ISBN 978-0-330-41929-1
- Taylor, Fredrick (2005), Dresden:Tuesday 13 February 1945, London: Bloomsbury, hlm. 202, ISBN 0-7475-7084-1
- Welchman, Gordon (1984) [1982], The Hut Six story: Breaking the Enigma codes, Harmondsworth, England: Penguin Books, ISBN 0-14-00-5305-0 An early publication containing several misapprehensions that are corrected in an addendum in the 1997 edition.
- West, Nigel (1986), GCHQ: The Secret Wireless War, 1900–86, London: Weidenfeld and Nicolson, ISBN 978-0-297-78717-4
- Wilkinson, Patrick (1993), "Italian naval ciphers", dalam Hinsley, F.H.; Stripp, Alan, Codebreakers: The inside story of Bletchley Park, Oxford: Oxford University Press, ISBN 978-0-19-280132-6
- Winterbotham, F. W. (1974), The Ultra Secret, New York: Harper & Row, ISBN 0-06-014678-8 The first published account of the previously secret wartime operation, concentrating mainly on distribution of intelligence. It was written from memory and has been shown by subsequent authors, who had access to official records, to contain some inaccuracies.