The Three Hostages: John Buchan

Spoilers! Stop reading if you care about such things.

The destroyer who by hypnotic suggestion can change a young socialite into a dance hall floozie and the heir to a great title into an inarticulate farmhand and a little boy into a little girl  is only to be countered and bested by his own weapons- and so a bunch of upright British heroes go up against him in masks: rock solid Richard Hannay becomes a dupe and a disciple, his wife flits from disguise to disguise and mercurial Sandy Arbuthnot (who I picture with the face of Alec Guinness) must turn himself into- well- lets keep that one a secret.

The certainties of Buchan's world are under attack. If a man who is so completely a sportsman and a clubman and an all round good egg can be plotting the otherthrow of civilisation as we know it then anything is possible. This is a book that expresses the most profound anxiety about personal identity- and prophesies the rise of humanity's newest enemy- the hidden persuader, the propagandist, the ad-man- and calls him a black magician- which is exactly what he is.

Also, of course, it's a cracking yarn.