Richard Thurlow begins his account of British domestic security in the 20th century with the usual polite statements about aims and intentions but one of these claims is worthy of comment. He states: "Like all recent writers on such matters I make the ritualistic but nevertheless true statement that I have never knowingly communicated with or spoken to a security or intelligence officer, nor indeed asked any civil servant for improper information."

Well, perhaps the reviewer ought to begin with his confession: "I have knowingly talked to many security and intelligence officials and I have asked for 'improper information' on numerous occasions." There now - it's out - what a relief!

Maybe I am wrong to treat this issue lightly but it does seem surprising that any historian of contemporary British government could contemplate writing a serious study of a significant government activity without conducting at least some conversations, if not interviews, with those involved. It is hard to imagine that relying solely upon the published record would be a credible historical methodology in any other field of contemporary history - why should it be seen as a virtue in this one? Obviously it is because the author believes that there is something peculiar about his topic that requires it to be approached only at the end of a very long stick.

What of the second part of his claim, that it has become common practice among writers on the subject to the degree that it has become a ritual? Is it a ritual only among a particular sect or is it an established part of orthodox doctrine? A rough check of recent books on my shelves suggests that it is only Peter Gill who uses much the same formulae, while Laurence Lustgarten and Ian Leigh state that a "few people spoke to us on conditions of anonymity". Bernard Porter states that he would not knowingly have spies or informers as friends and that one should not be gullible enough to believe that all one's sources tell one, but he does not claim not to know any spies or not to have talked to any. Other writers, far from making such claims, publish books and articles full of footnotes to interview material. I am therefore far from convinced that this has become a "ritual" among all recent writers.

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This issue is far from a trivial one because it tells us a great deal about Thurlow's attitude to his subject. It indicates that he was wary, perhaps even fearful, of what he would find - right-wing conspiracies, paranoia, abuses of power, excessive and secret influence on government and all the other possibilities of the pantheon of security intelligence failings. In fact what comes across in the book is that although there were individuals who suffered from some, perhaps even all, of the expected failings, most of the government, for most of the time, did not. For example Thurlow continually points out that there were important suspicions about the whole business of domestic espionage at senior levels of the civil service and among politicians.

He states: "However, the golden age of liberalism left behind it important residues that ensured that the old prejudices about intelligence and the work of secret police officers biased national perceptions of both these activities."

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Later in the work Thurlow states: "Somewhat ironically the failure of Goronwy Rees to get the British authorities to believe his claims about Soviet spy rings shows, in retrospect, that the establishment was unwilling to face up to the fact that infiltration was a serious problem, and that unsubstantiated accusations and McCarthyite smears were seen as a greater evil than the possibility of renegade upper class treason."

I do not believe that these quotations are unrepresentative of his conclusions although I do accept that he finds certain individuals such as Sir Basil Thompson (director of intelligence) and "the most right-wing home secretary of the inter-war years, Sir William Joynson-Hicks" conform, at least in part, to the expected pattern.

This pattern, his expectations not being fulfilled, begins to break down for the more recent past. Thurlow's interpretation is that Britain has finally accepted the institutions of a secret state and has abandoned its liberal tradition. There are, however, two other possible interpretations which need to be considered. One is that this picture is a distortion due to the fact that official records are simply not available and that had he been writing in the 1950s about the 1920s or 1930s he might have come to the same sort of conclusion.

The other possibility is that the world in which security intelligence operates has changed such that the risks to security are measurably greater. The loss of innocence need not reflect the loss of liberal values but the rape of the United Kingdom by terrorism.

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It may seem ironic in the light of what I have said that, although Thurlow seems to have had clear expectations, he does not have a clear theme or thesis. This means the value of the book lies more in the range of topics covered and the range of Public Record Office files examined than in its picture of the nature and activities of "The Secret State".

It is more a chronology of how parts of the secret state reacted to events rather than an account of how these agencies evolved. There is, for example, no organisational chart, little discussion of the internal bureaucratic politics associated with security intelligence, no discussion of recruitment or training, and no discussion of the mechanism, or the lack of mechanism, for government assessment of security information. It may be that the files do not provide answers to some or all of these questions, but we glimpse the secret state through its reaction to events rather than as the central player in the book. However, the book does provide a great deal of valuable information and it is to be welcomed as adding to our knowledge of this important aspect of 20th-century British history.

Kenneth Robertson is senior lecturer in sociology, University of Reading.