The sinister, murky world of espionage is laid bare in this revised and updated edition of Philip Knightley's powerful book about spies and spying in the 20th century.

One its most significant "characters'' was William Le Queux, the British author, whose fantasies about German spies was a direct factor in the creation of the first bureaucratic intelligence service in this country, in the years leading up to the First World War.

Germany, France, Russia and other powers joined the rush towards espionage, followed by the Americans, who now have the largest and most costly intelligence service in the world.

The problem with buying information from spies is that it can tempt the informant to lie. Even when valuable facts have been obtained through espionage, they have often been disastrously ignored by those who should have acted.

Knightley reminds us that intelligence services have a vested interest in their own survival, so must constantly postulate some or other threat to the nation.

The Second Oldest Profession by Phillip Knightley is published in paperback by Pimlico, priced £14.99.