http://gen.lib.rus.ec/book/index.php?md5=914A36263B1F4AB1E28DF9C8C7085084
>J. C. Masterman - Double-Cross System: The Incredible Story Of How Nazi Spies Were Turned Into Double Agents
>Masterman himself wrote the book without revealing anything about Ultra, still highly secret, that greatly aided the double-cross system. The ban on Ultra would be lifted in 1974.
still worth reading
https://archive.org/details/ultrasecretthe00fred
>Frederick William Winterbotham - the ultra secret (Open Library)
>the first popular account of Ultra to be published in Britain