The book draws heavily on Mr Knightley's series of interviews with Philby spread over 6 days in Moscow towards the end of Philby's event packed life. Obviously, it also pulls on other sources and, as a result, succeeds in painting the most rounded available portrait of the KGB Colonel. The story is told in chronological order and takes in the major (known) episodes: life as a Cambridge student in the '30's, journalism in Spain during the civil war, SIS, Beirut and finally - almost inevitably - Moscow, the centre of the USSR and Philby's political home.

The problem that any writer faces when writing about spies, defectors and their ilk is attempting to distinguish the facts from the disinformation, the misrepresentation, the conjecture and the simple errors. Mr Knightley does an admirable job in getting out the most plausable and persuasive story. Of course, there are loose ends (e.g. were there really other highly placed but as yet unmasked moles in Bristish intelligence?)but it seems that this is the fullest account we are likely to get. Mr Knightley appears to have no axe to grind, although he is not coy when it comes to expressing his own view on the murky world of "Intelligence".

So, why does Philby continue to fascinate? Simple really. Its a cracking story yet raises important questions on the human condition. Traitor or Idealist? Depends on where you are standing.