The Three Hostages

John Buchan is best known as being the creator of Richard Hannay in "The Thirty-Nine Steps". 
"The Three Hostages", first published in 1924, is his fourth of the 5-book series of the Richard Hannay novels.

After distinguished service in the First World War, Richard Hannay settles into peaceful domesticity with his wife Mary and their young son. However, news comes to him of three kidnappings. With no more than a few tantalisingly cryptic lines of verse as clues, he is soon on the trail of Dominick Medina - a charismatic polymath but a man 'utterly and consumedly wicked'. As Hannay uncovers an international plot to twist innocent minds through hypnotism and blackmail, it appears that he has met his match in one of Buchan's most memorable villains.