Fascinating … [Ferris] has rescued several great women codebreakers from obscurity … [Bletchley Park] has become embedded in national myth, but Ferris offers cool and balanced judgment … This monumental work completes the authorised picture of a century of British intelligence, a testament to how far Britain has moved away from the cult of official secrecy

Ben Macintyre, The Times

In Ferris we have a shrewd and scrupulous historian … The references to individual people at all levels of the service are many and illuminating … Small details can bring a nod or a smile when one is reading … GCHQ shows it is alert to the role of a security and defence agency in a modern democracy, and Ferris is to be congratulated for shedding so much light upon it

Vin Arthey, Scotsman

The book is at its best when sifting the role of signal intelligence (Sigint) in the Falklands war and other late imperial conflicts such as Indonesia and Palestine … Comprehensive

Luke Harding, Guardian

GCHQ emerges from the shadows … The story of the codebreakers is in fact a parallel history of the entire twentieth century … There is intriguing detailing of the organisation's structure and systems ... Illuminating ... Absorbing

Sinclair McKay, Spectator

What happens when a tiny caste, so obsessed with keeping thing hidden that it speaks its own language of Ultra, Venona or Zircon, opens up? … The answer – not withstanding significant restriction on what Ferris was allowed to publish – is a book of revelation … Although he spent months sifting the papers in a high-security Cheltenham vault, [Ferris] does not lose sight of the big picture … There is much in the book that illuminates other aspects of postwar history, from the struggle against the Jewish underground in Palestine to the 1982 Falklands conflict … Today, [Ferris] argues, greater openness about intelligence gathering does not affect its relevance and power. His book is an example of this, and shows that the abandonment of Cold War levels of secrecy about GCHQ benefits us all

Mark Urban, Sunday Times

There is so much more to this secrecy-shrouded outfit, reveals Canadian historian John Ferris … Fielding formidable research, Ferris tells a global tale of mathematics, engineering, data sciences and linguistics in the service of politics, diplomacy, war and security

Andrew Robinson, Nature