The Three Hostages (Richard Hannay #4)

After the war and newly knighted, Richard Hannay is living peacefully in the Cotswolds with his wife Mary and son Peter John. Unfortunately, a day arrives when three separate visitors tell him of three children being held hostage by a secret kidnapper. All three seem to lead back to a man named Dominick Medina, a popular Member of Parliament. Hannay uncovers a dastardly plot involving hypnotism and the black arts, as well as the more earthly crimes of blackmail and profiteering.