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Book Review If information indeed is power, then GCHQ is undoubtedly the closest thing the British government has to the Death Star.

As the historian Richard J Aldrich notes in the introduction to his excellent new history of the Cheltenham-based agency it represents by far our largest, most expensive, most productive - and yet most secret - intelligence effort. GCHQ: The uncensored story of Britain's most secret intelligence agency instantly ranks as the most essential exposition of the hidden power wielded for 70 years by Britain's information superweapon.

Indeed, apart from its heroic beginnings at Bletchley Park during World War Two, GCHQ's presence in the popular consciousness is limited to vague paranoia and some awareness that what it does is frightfully clever and terribly secret.

The agency's success in maintaining not just its secrecy, but a helpfully low profile, is best demonstrated in the book by its exposition of the intimate relationship between GCHQ and the US' National Security Agency (NSA).

How many appreciate that when politicians and journalists reflexively reference the "special relationship" between Westminster and Washington they are really talking about a Cold War bond originally forged between eavesdroppers in Cheltenham and Fort Meade, the Maryland base of the NSA? It's a historical paradox that as the Empire was dismantled in the 1950s, Britain held its seat at the top table by dint of signals intelligence from vestigial remote territories such as Diego Garcia and Hong Kong.

Aldrich packs much of his book with ripping operational tales from the Cold War period, sharply written with due attention to the real people involved, as well as the politics. The high stakes of these exploits is not forgotten either: in one disastrous episode in 1972 three GCHQ staff were taken hostage and murdered by Marxist rebels in a raid in Turkey.

The pivotal role of GCHQ in the history of our relations with the US is expertly explored in a chapter on transatlantic tensions in the early 1970s. American attitudes were at the time described by a British diplomat as "prickly and difficult" as a result of the Watergate affair. That Nixon's administration was brought down by its own amateurish surveillance operations surely raised wry smiles in Cheltenham.

Aside from 20th century giants like Nixon and Kissinger, the book's most memorable character could just as easily be a villain in a schlocky Cold War thriller. Geoffrey Prime, an RAF veteran who got a job as a GCHQ linguist in the late 1960s, at the behest of the Soviets, was responsible for what still ranks as the agency's worst known security breach. Aldrich recounts the bizarre tale of microdots, invisible inks and briefcases of cash with commendable restraint. Prime's eventual arrest in 1982 not for espionage, but for sexual assaults on children, provides its own rotten drama. That GCHQ strongly resisted the investigation for fear of embarrassing itself in the eyes of its American friends further sours a distasteful episode.

Events surrounding GCHQ's other famous leaker, Katherine Gun, are also well covered. She revealed to a journalist in 2003 that Cheltenham planned to eavesdrop on UN Security Council members as part of transatlantic efforts to secure a resolution to legitimise an invasion of Iraq. Sir David Pepper, GCHQ's last director, has said he was as shocked by Gun's security breach as by the unmasking of Geoffrey Prime.

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