John Buchan 
Three Hostages 

After the War, Hannay is married to Mary and living peacefully in the Cotswolds, when he receives a request to help solve the mysterious kidnapping of the children of three prominent people. Given nothing to go on but a few mysterious clues, Hannay, assisted by friends like Sandy Arbuthnot, must track down the dastardly villains behind the plot before it’s too late… (Wikipedia)