Spooks: The Unofficial History of MI5

This is a solid book, ambitious in the scope of its content and thorough in every aspect of espionage in Britain, from the creation of MI5 in 1909 to our present world of Big Brother surveillance, Orwellian doublethink and anxieties about counter-spies, cells and "sleepers".

The text is small and the word count per page is massive. With tiny margins, this makes for a demanding read, but it is worth it. The general reader would perhaps not require such detail, and arguably the academic specialist is the reader the authors have in mind.

Thomas Hennessey and Claire Thomas may confidently lay claim to having produced the first general history of MI5, and there has been a demand for such a book ever since archives were opened to researchers. However, in some ways there is too much information here.

It is, in essence, two books combined: one dealing with the story through the biographies of personnel - from Vernon Kell at the beginning to today's director general - and the other giving a factual account rooted in archival sources. This is because there is a huge amount of reference data, integrated into the central narrative.

In fact, this is a reference work. There is such a mass of detail at times that the reader has to pause, reread and absorb a large quantity of information, mostly names and places. Every phase of the development of MI5 is here in depth, from Kell's liaison with Scotland Yard and Special Branch through to the attempt to combat the new terrorism of the 21st century.

Perhaps most exhaustive is the series of chapters dealing with the Second World War, after the sacking of Kell, and Sir David Petrie's reforms.

Before Kell and the Committee for Imperial Defence at the opening of the 20th century, espionage had been present only in the dark margins of the empire. It was a patchwork involving various categories of men, including diplomats, travellers, journalists and military staff.

The Arab Bureau under Mark Sykes and others in some ways instigated certain concepts of structure and organisation of intelligence in time of war. However, it took Kell's work against the German spy network on the domestic front during the Great War to reveal the true potential of espionage in all contexts, from home security to international liaison.

Hennessey and Thomas interweave stories of the human element within the larger meta-narrative of espionage becoming placed and defined within the state. For instance, they use the memoir of Kell's wife to provide a telling insight into the personal side of this talented organiser and linguist.

There are hundreds of cameos of agents as the story progresses, and some are outstanding, such as those of "Tin-Eye" Robin Stephens, the interrogator of internment in the Ham centre near Richmond, and the mysterious Miss X, who played a major part in breaking the Glading spy ring.

But arguably Maxwell Knight steals the show: this eccentric and brilliant man, known as "M", worked out of Dolphin Square in London, running a team of infiltrators into Soviet-linked groups. From this, Olga Gray emerges as a woman whose life will surely be featured in a film soon, with the story of the Woolwich Arsenal spy ring.

At the core of the book is an education in understanding the key elements of this so far little-known history.

We have had excellent books from Nigel West and Andrew Cook on the years up to 1945. But since the National Archives published its guide to sources last year (British Intelligence: Secrets, Spies and Sources by Stephen Twigge and Graham Macklin), historians of espionage have known what is there. Now Hennessey and Thomas are to be congratulated on their definitive and comprehensive history.